The tldr is that they don't know it's alphabetic for sure (see below quote). The main scholar (Glenn Schwartz) who co-oversaw the '94-'10 excavation isn't an expert in writing. He put it out there around 2010 and said "maybe it's alphabetic, idk" and there was not much followup from the community. So he consulted with some writing experts who helped him with the 2021 paper where he goes over the evidence for different possibilities and suggests that the strongest argument is for alphabetic. The dating seems to be on firmer ground but the error bands on this and Wadi el-Hol can probably knock a century or two off the "500 years".
A decent summary is the blog post below from another researcher who briefly was part of the same dig and a former student of Schwartz (so not entirely independent):
http://www.rollstonepigraphy.com/?p=921
It is worth noting that in the past Schwartz has been reluctant to affirm that the four inscribed clay cylinders from Tomb 4 of Umm el-Marra are alphabetic (Schwartz 2010). Thus, he certainly did not rush to this conclusion. Moreover, his most recent article about these is also very cautious (Schwartz 2021), as he moves through various possibilities (as discussed above). But it is clear that he is now willing to state that this is the most reasonable position (i.e., it is Early Alphabetic). And I concur. That is, the most reasonable conclusion is that the Umm el-Marra clay cylinders are inscribed with signs that are most readily understood as Early Alphabetic letters (graphemes). Moreover, since the Early Alphabetic alphabet was used to write Semitic, it is logical to conclude that this is the language of the Umm el-Marra inscriptions (the fact that they were found in Syria would also augment this conclusion, of course).
The full blog post is worth reading and summarizes the case for various non-alphabetic possibilities.
I admit I didn't have time to read this blog post deeply, but it doesn't sound very convincing. It doesn't bring any EVIDENCE that this is an alphabet it just cites other cases of possible alphabets in Mesopotamia and the near East [1].
Besides that, this blog post mentions some morphological characteristics of the inscriptions that make the author believe the writing is alphabetic, but it fails to mention these characteristics. I don't doubt Rollston has good reasons for this statement, but the claims behind them need to be published and reviewed. I'm not sure if this is the case (and I do not have access to the 2021 article).
[1] This includes the Lachish Dagger I tried to look up, but its dating seems disputed, but even the earliest proposed date (the 17th century) is more recent than the Wadi el-Hol inscriptions, so I'm not entirely sure what it is supposed to prove, except perhaps an earlier spread of the Alphabet from Egypt and the Sinai peninsula to Canaan proper?
> it doesn't sound very convincing
That's because it's not a strong conclusion. It's a "better than the alternatives" hypothesis. Repeating my tldr above "they don't know it's alphabetic".
> doesn't bring any EVIDENCE .. some morphological characteristics of the inscriptions
I'd say the "morphological characteristics of the inscriptions" count as evidence and I'll just recap everything linked that I think counts as evidence: the graphemes include several repetitions even with only 12 signs in total; they don't resemble cuneiform at all; they have a weak resemblances to some Egyptian glyphs but weak and Egypt didn't have these clay cigars; they have a weak resemblance to some Indus glyphs and (later) Byblos glyphs but again weak; they don't appear to be numbers, potmarks, etc.; but what they do strikingly resemble is later alphabetic signs, to the point where the author, one of the foremost experts on Semitic epigraphy, really wanted the dating to be wrong.
Now the blog post doesn't go into much detail on these items but Schwartz's 20+ page 2021 paper (I had no trouble getting a free, legal copy) does (not always a lot more detail but also covers more possible alternatives). But, like the blog post says, the case Schwartz 2021 makes is still extremely cautious and he basically concludes that we just have to hope we can find more examples to confirm what kind of system they are from, and to increase the chance of deciphering them.
> Egypt didn't have these clay cigars
Sometimes a cigar is just a cigar.
I'll escort myself out now...
How is it better than the alternative "we have a set of symbol and we don't know what it means"? I really think there is a merit in saying "with this sample size, every theory we put out has low confidence level".
With 12 signals in total, it's very hard to show patterns that are in line with an alphabet. I don't think that with this sample size you can make a very strong claim that the chance that this is an alphabet is higher than the chance is that these symbol serve any other kind of purpose (including being a non-language). The main claim seems to be that repetition (what kind? I'm a quite disappointed the blog post has no transcritions, considering it's just 12 symbols we're talking about!) makes the chance that this is an alphabet higher. The rest of the claims (it doesn't resemble cuneiform, doesn't seem to be derived from hieroglyphs and doesn't seem related to any other script) are meaningless. The resemblence for later Canaanite alphabetic signs is interesting, and could probably be more convincing if we had a larger sample size.
So in the end, if we are convinced by these claims, we're basically saying something like "We have at most 1% confidence for every other theory, but we've got 2% confidence that this is an independent development of the alphabet that may have inspired the Canaanite alphabet we've seen 500 later". Higher confidence that is still far below the threshold doesn't cut it.
Now, I'm pretty sure the original article did not put the theory in these terms, but the headline is somewhat sensationalist, and the way it was picked up in newspapers is even worse, for instance:
Scientific American: World's Oldest Alphabet Found on an Ancient Clay Gift Tag
Stopping the press from misreporting science is a bit like trying to stop space rockets in midair with your bare hands, but even "Evidence of oldest known alphabetic writing unearthed in ancient Syrian city". The popular understanding of the word evidence is assumed to be "hard" evidence by default, not a weak evidence that bumps up the probability of a certain theory a little bit more.
I'll actually be quite excited if this turns out to be truly an alphabet encoding a Semitic language (it opens a lot of interesting questions and possibilities), but I'm not holding my breath for it.