However, the 'X' they chose is "just" a single UTF8 character, which would then make it classed as a _standard character mark_.
Hmmm.
The Meta trademark is for this design: https://tsdr.uspto.gov/img/87980831/large?1690302463976
It's true that they use the text "X" in the (textual) description of the trademark, but that is not what's being trademarked. See the "Mark Information" drop-down on the page[1], for more detail.
[1] https://tsdr.uspto.gov/#caseNumber=87980831&caseType=SERIAL_...
However, the 'X' they chose is "just" a single UTF8 character, which would then make it classed as a _standard character mark_.
Hmmm.