Earlier quoted context omitted.
Not necessarily. It their canonicalization function were idempotent (e.g., the identity), then this database scheme would work well. How else do you map username to user id?
Well generally it makes sense to use that userid as a primary key everywhere rather than the username. You only need the mapping of username to userid in one place. Their current scheme also makes it sound hard to change your username.
Ah I fully agree. I misunderstood your post to say you should never use a user name as a key.