They seem to suggest doing it that way because the URL of a user profile is going to be guaranteed to be unique, and can only be owned by the owner of the domain.

Immediate design issue right there: the URL of a user profile is not guaranteed to be unique, and while it can “”“only”“” be owned by the owner of the domain, 1.- it’s not owned by the user of the profile and 2.- the ownership by the domain owner is revocable by a third party.

Design-wise, it feels to me like they decided that land / house deeds could be certified by municipal traffic signage.