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Take a look at
this thread. As far as I can tell, those cases are related to "deleted" profiles (by an Admin), where the deletion is not physical, but rather result in the profile being anonymized (whilst preserving the posts).
Exactly like that, I agree with you.
Not sure if there was a time where one could actually post without creating an account on older SMF versions.
I've never gotten one and probably don't have one. DdmrDdmr, I'm sure guests can only view and move from thread to another thread to read (reading and enjoying some drama), but they can't post without an account. That makes sense.