Ah, but how do you trademark a Monotype capital X when the glyph already has design protection and is part of Unicode?

You do it the same way everyone else trademarks stuff? I don't see how using an existing font for the logo or the fact that it's in Unicode makes any difference here. A logo just has to be distinctive compared to other existing trademarks, it can be made from elements that are common in other contexts.

For example, Mastercard's logo is just a red and a yellow circle overlapping each other. Colored circles are nothing new or unusual.

They’re taking someone else’s creation and saying “when used in this trade context, it can only belong to us.”

Yes. That's how trademarks work. It's exactly what they're for.

The thing that's the stumbling block for Twitter's rebranding is that "X" has already been trademarked by other companies, including in the context of social media. Not that it's in Unicode.