> Mark Drawing Type: 1 - TYPESET WORD(S) /LETTER(S) /NUMBER(S)
They do not own the trademark for the letter "X" (that would make a "Standard Character Claim") but the very specific typeset, as featured on the right side of the linked page.
Here [1] it patents a specific illustration format of X. Again no standard character claim.
So no issue for former twitter, as you can easily see the typeset is very different.
I do not know if there is any precedent of a standard character claim for a single letter, but that would really sound ridiculous to me tbh.
[1] https://tsdr.uspto.gov/#caseNumber=87980831&caseType=SERIAL_...