I've never heard pseudo used as a noun - must be a neologism.

How can a handle be an anonym if anonym means lacking a name? I don't think a forum handle is either a pseudonym or an anonym. I don't think anonym exists at all as a word - you can't have a nameless name; and a handle is not a false name, it is usually a nickname. Sometimes it can even be your real name! A pseudonym is a real life name not your own that you take on for reasons of anonymity (eg a celebrity wanting to avoid paparazzi) or for nefarious reasons, or if you are a spy, etc. The Pook is not a pseudonym because it is not a name at all.