Your correspondents commit fundamental errors if they argue about "stages" of revolution/evolution whether socialism and communism are separate modes of production or not.
Marx and Lenin were adults at different times and in vastly different historical circumstances - had they been contemporaneous they may have violently disagreed on analyses/tactics.
Marxism, as I understand it, is at its most practical as a tool kit for analysis and action - not as a biblical tract where a template can be pulled out for any occasion and metaphorically dropped over the issue in question.
In Russia after Lenin's death - and during his protracted illness - Stalin was busy canonising Lenin and turning his writings into holy writ.
The (necessary) tactic of socialism in one country at a time when the Soviet Union was besieged by enemies without and within is not necessarily the greatest tactic today.
There's nothing wrong with using apposite quotes from the Marxist classics. The great intellects of the Bolshevik revolution and after analysed the shape-shifting clothes and forms of the beast - capitalism - while it always retained its prime directive of profit maximisation.
Getting locked into stageism merely echoes the Jesuit question "how many angels can dance on the point of a needle?" The Marxist toolkit becomes the manual itself.