Trademarks are against a "class", these can be quite wide. They would clearly want to own that exact symbol for "Class 9: Computer and software products and electrical and scientific products."

Unicode already contains exactly it, it's a mathematical/scientific symbol, even if they did get the trademark in that category (which I doubt) they will never be able to enforce it. With trade marks, if you don't successfully protect them, you loose them.

It’s totally enforceable. It’s a trademark. Who else is doing business with the Unicode doublestruck X?

I think this would seriously confuse his core audience, so I doubt that’s the intention. Then again, I think rebranding Twitter to X is a terrible idea, so what do I know.

It is a problem, because if theoretically Musk were to trademark that character (improbable), Unicode would likely be forced to remove that character and break every instance of it across the internet. Not to mention that anybody using that symbol, which already has an established meaning in mathematics, would be infringing on that trademark.