Trademark are there to reduce confusion in the marketplace so it’s not about the look of X alone but also the products involved. Acura’s A logo is very distinctive but Audi couldn’t just swap their logo an A in a circle using a different font and be ok. They are both car companies so having different A logos would be confusing.
Where the specific design of the logo is critical is for shirts etc. If you’re selling clothing with other people’s logos that’s a problem, but a distinctive X design is going to be fine.
This is (to my understanding) the difference between a character mark and a design mark. The Acura logo is a design mark. Acura don't, AFAIK (but I haven't searched because searching the USPTO for "A" sounds like hell) have a character mark on the letter A for use in cars. Consider the counterexample in the same industry: Honda and Hyundai both have H in a frame as a logo, and I have no knowledge of a trademark dispute between the two.
The X trademark in question appears to be a Service Mark[0], meaning it is a name to represent a service (in this case some kind of social media chat service). My understanding is that the trademark applies to competing services regardless of the artwork or logo, so the social media chat service formerly known as Twitter may have trouble avoiding a collision with it if they want to rename themselves X.
Yes, though my point about the circle should have been more clear. I think Audi could be fine with an A inside of an triangle because it would be plenty distinctive.
Similarly the Honda and Hyundai logos use very different frames rather than the common circle which makes them far easier to distinguish than different fonts inside a circle. Namely a box with rounded corners vs an oval. Honda’s H also floats in the frame where Hyundai connects to the oval etc. The point of a logo is to be distinctive and they are.
Where the specific design of the logo is critical is for shirts etc. If you’re selling clothing with other people’s logos that’s a problem, but a distinctive X design is going to be fine.