Fair point, I think it depends on the certainty. I am just about 100% certain Neil Armstrong was the first human to walk on the moon, so "first" makes sense. Hillary and Norgay were the first "confirmed" climbers of Everest. The "confirmed" part is needed because it is reasonable that others before them made it without evidence. I am open to the possibility a civilization had a writing system before the Sumerians, and the evidence has been destroyed or not found yet.
I'm more than open to the possibility - I think it's very likely that something existed beforehand which we don't (yet) know about.
Right, but that's recent information. When talking about deep history, "that we know of" is effectively a given. There will never be true certainty.