Doc Fritz;161615
Technically, that's only a fan speculation. One that I like a lot, since I was the one who came up with it, but at the end of the day it's only that–speculation. The show itself never explained the Stay Puft in the Containment Unit, and why it acted so differently than the one in the movie, which was definitely Gozer.
Oh, I think it's pretty well established in “Revenge of Murray the Mantis” (the first episode I ever saw, so I'll keep mentioning it ‘til my head explodes). Egon opens the ECU, and Stay-Puft comes out.
PETER: We released the big guy.
DET. BULLOCK*: The Stay-Puft Marshmallow Man? Are you freaks nuts? Don’t you remember what happened the last time he was here?
PETER: Yeah, but he's all better now…we think.
(*I don't know this guy's official name, so I'm calling him “Bullock” because that's what he looks and sounds like.)