Oh, I guess in Marx's vision the transition period happens outside of society. Hey guys, let's put society on hold and do a transition period really quick.
the reason lenin started differentiating between "socialism" and "communism" was because of left liberals in his time who were co-opting the socialist label and so he felt it necessary to point out the difference between "socialists" who really just favoured national chauvinist social democracy, vs communists who genuinely understood the marxist theory of history. even a cursory reading of lenin will inform you of this, but of course these radlibs are unfamiliar with any of the theory they lob so much criticism at.
Whenever this gets dragged out I have to point people towards Marx's Conspectus of Bakunin’s Statism and Anarchy which basically reads like a reddit thread where Marx quotes Bakunin bit by bit ripping him apart.