Musk, for all his genuine achievements, is also a bullshitter. If/when his FSD prediction fails to come to pass, I expect that he will just say "Well, when I said FSD, what I really meant is..."
That is already happening. Go visit the Tesla forums, and you will hear a lot of fans trying to tone down expectations for FSD, redefining what that term means. I would have thought "full self-driving" was pretty explanatory, but from things I've recently read, the goalposts have already shifted significantly.