Eh, I dunno that I'd actually characterize him as a liberal so much as him being an authoritarian that just pushed whatever happened to serve him at any point. Kinda in the same vein of fascists not having any economic ideology, just whatever serves their ideal of the state at any given moment. So yeah, I certainly agree with your sentiment that Stalin certainly was not a communist, but more because he only cared about gaining/maintaining power rather than actually subscribing to any economic theory.
Stalin was by most accounts a true believer in Marxism-Leninism. There are simply too many unforced ideological-caused errors for it to be anything else, like Lysenkoism rejecting Mendelian genetics because the fascists loved it and it raised questions of eugenics.
The Stalinist economic plans were thoroughly Marxist. The problem there (as far as Stalin's true believer status is concerned) was not ideological.
He was just a drunk, paranoid, murderous asshole. Socialists simply don't like admitting that they're as vulnerable to hypocrisy and corruption as anyone else.