This is a legitimate threat model. I'm not sure why you think it wouldn't be. Spies and others do use fake identities. The threat model is that there is that there is a way to determine who is faking their identities.
A picture is a username... Are you trying to say that your face isn't personal identifying identification (PII)? I'm not sure what your argument is here, because it can't be that. That argument would be absurd, so I'm sure I am misunderstanding.