My comment was mostly hoping only to make a generalised point; although open source software often has certain (good) features, simply being open source doesn’t mandate or guarantee any feature at all.

On the username subject in particular then absolutely; an inability to change your username is not the same level of ‘bad’ as things like lock-in or spying. As other commentors have mentioned I’m sure it’s not intentional at all, and simply a consequence of the implementation difficulty with federating that sort of change.

Like the same way you can’t ‘change’ your email address. The email address is your identity - if you want a different one, you need to make a new one.