He didn't submit a preferred username to thymos; he simply stated that he would be fine with any random username as long as his real name was removed, and the admin fixed that up with some random letters, which I'm sure in the next 2-3 years when he becomes a more established member, he will request a name change because this is not something anyone can pronounce or remember while quoting.
He didn't exactly say that he would be fine with any random letters as he even gave a suggestion so they could have changed it to that rather than this "mxfwzzwizqbz" due the reason you mentioned in your post.
I don't care what you guys change it to, you can change it to Michael23913 or something like that but please I just want to remove my last name from my account. I don't want my full name on this thing lmao
I guess theymos chose the first option because he clearly stated that he wouldn't mind if the admin changed it to some random shit and that's exactly what thyemos did since he wanted to remain hidden from the previous username. No one will be comfortable with this bot-generated username, and I'm sure he'll request a name change sooner or later. This is one of the worst forum usernames I've ever seen.