It's true that the nonexistence of the God of Abraham is technically unknowable. But I think saying so - and emphasizing that point specifically - actually ends up being misleading in this case. Because ultimately, everything is unknowable at some level. But we don't hold most statements to the same level of scrutiny as 'there is no God'.

If a family watches The Wizard of Oz together, and afterwards, the youngest child is scared because they're afraid of the Wicked Witch and her flying monkeys, the parent may tell them not to worry because "the witch and the monkeys aren't real". Now, the parent can't actually prove their nonexistence. But does that make the statement dishonest? Would it be more honest if the parent admitted their ever-so-slight deficiency of absolute knowledge, and instead said that they were 'pretty darn sure' that the witch and monkeys didn't exist? I don't think so. I think it's more clearly understood by the child - and by extension, more honest - if the parent simply says that the witch and monkeys don't exist.

By the same token, I don't think it's dishonest for an atheist to say that there is no God of Abraham. In the same way it wouldn't be dishonest for an atheist (or a Christian, for that matter) to say there is no Zeus. Because the likelihood of Zeus existing is so wildly unlikely that it's simply not practical to beat around the bush - so we just say Zeus doesn't exist and move on. Why can't we say the same thing about the God of Abraham?

On the other hand, I do think Christians oversell their certainty when they claim to know the truth though. They are basing their beliefs on stories that are thousands of years old, with no present day way to validate most of their claims. And that, to me, seems misleading - and not in a nit-picky technical way, like the slim chance that atheists may be wrong. But in a real, obvious way that inhibits communication, collaboration, and progress.