This does not make GP incorrect though. It just means we really cannot know for sure how the writing system works until we have enough information to decipher the inscriptions.

I don't take beef with the possibility of an earlier alphabet that predates the Proto-Canaanite alphabet — that is entirely plausible. But I think the article is overselling the story. The evidence is not very strong at this point, and I although I can be wrong, I suspect it can never be with if we remain with just four very short inscriptions without external context.

It is important to clarify the vast difference between this and the decipherment of Egyptian Hieroglyphs. I think the myth and magic of the Rosetta stone is overemphasized in popular culture, so just a few points of difference between the Egyptian Hieroglyphs and scripts like the Indus Valley Script or Linear A.

- Of course, to start with we did have the Rosetta Stone, and we have no equivalent for these scripts. But the Rosetta Stone was rediscovered in 1799, while Champollion provided the first phonetic interpretation of Egyptian hieroglyphs only two and half decades later, in 1822. But even after Champollion's famous achievement, we weren't able to read most hieroglyphic texts yet! Champollion didn't realize that many phonetic hieroglyphs represent not just a single consonant, but often two or three different consonants! It took a couple of more decades until we Egyptian was fully deciphered.

- We knew exactly which culture and language the Egyptian Hieroglyphs belonged to. More importantly, we had a vast wealth of external historical sources about this culture that we could read: mostly in Greek, Hebrew, Roman and Aramaic. From these sources we knew the names of Egyptian kings that we could expect to find in Egyptian hieroglyphs, and we knew enough about Egyptian culture, religion and history to often guess what the texts would be talking about. This is not anywhere nearly as true for the Indus Valley Script! Since we don't know who their kings were, we cannot use the names of kings as a highly verifiable way to test the phonetic writing hypothesis.

- We had a vast quantity of Hieroglyphs inscriptions. There are fewer attested Indus Valley Script inscriptions, but the number should be enough if we just had other external clues.

- Egyptian still had a (barely-)living descendant (Coptic) at the time Champollion and other scholars were working on its decipherment. Coptic priests and AFAIK even native speakers have provided a lot of help them in understanding how the Egyptian language they were trying to decipher might sound and work.