I wonder if he would even make it to the point where you would be able to pay somewhere/someone with his app. Hard mode: don’t search for places by accepted payment methods. With his current reputation it can be tricky. Who would even want to be banned from payments because he posted something he didn’t like on Twitter?
The letter by itself, maybe, but maybe not. The letter in a distinctive font, though (which is what they're doing), very likely.
Remember the purpose of trademark is consumer protection: to be able to tell that a product or service really is from who you think it is. As long as the trademark is distinctive enough in the product categories it applies to, it's allowed.
But it’s not actually “X”. The symbol while obscure is called “ Mathematical Double-Struck Capital X “. Graphically the logo seems completely identical to the Unicode character. I don’t know if they can register it as a trademark it but would sure seem weird if you could that with something like “ ∫ “
Even the logo is mistakable.