Technically he's pseudonymous. I don't know if he always had the (fictional) last name "Branwen", but I have records of "gwern" (all lowercase) going way back. And yes I'm pretty sure it was the same person.
He says he has collaborators under the "Gwern" name now, but the main guy is the main guy and it's unlikely he could hide it.
How many citations for "Branwen 2018" are on the ArXiv now?