It's not incorrect, but a strawman (I didn't downvote you btw).

The question was why UTF-8 in MySQL has weird limits. It wasn't about what MySQL should have done in an alternative universe.

Yeah, but I didn't point out what mysql should have done in an alternative universe. Mysql DOES support fixed lenght unicode, just use the correct encoding: ucs2 or utf32. What it does with utf-8 is what any system that supports utf-8 must do.