Elon undoubtedly has folders full of cool images he's saved, so while he was still focused on the "X" idea, he rummaged around and found that one and thought, "Yeah! This is gonna be sick dude!"
Thus are decisions made by the world's richest teenage edgelord.
Let me get this straight; you actually think Elon Musk, himself, combed through images he personally saved, found the X, and personally submitted it as the logo replacement.
Elon Musk. The crazy douche billionaire. You really think he sat at his computer and spent time doing that.
You don't think, maybe, just maybe, a man with more money than God might have gotten someone else to do it? I can see him being out of touch and not caring enough about quality to say "go with that one" without checking where it came from. But I just can't see Musk himself looking through fonts to find an "X" that he likes.