I think, the main problem is that “X” doesn’t look like a name.

When someone’s not starkly aware of the platform being called that, they might think the author typoed.
Or is using it like the idiom “they posted it to X, Y and Z” (so just a nondescript set of platforms).
Or genuinely means the letter X and that just doesn’t make sense in the context presented.

“X, formerly Twitter” is just a better name than “X”, because it is recognizable.