The main forms of intellectual property are trademark, copyright, patent, and trade secret, and they all have significant differences from each other that cause no end of trouble when they get mixed up in common parlance. It's not uncommon. :)

I just did a little Googling and it seems that there's nothing inherent wrong with trademarking numbers, Intel lost their trademark to the 386 because AMD was able to clone it and a lawsuit determined that AMD could call their clone a "386 chip" because the term had become genericized. That's one of the things that trademarks are susceptible to that other kinds of intellectual property aren't.

It's not directly relevant to Apple's app store, but since we're on the subject of trademarks it seems that "X" is already trademarked by a number of companies, including for "social media services" by Meta. It's rather obvious at this point that Musk didn't make this decision with the sort of advance preparation that he should have.