"For most of its history, nearly everyone in the USSR was a farmer, so not proletariat." They certainly were in the context we are speaking of here. Official Soviet terminology, apparently starting at least from Lenin but I haven’t checked this thoroughly, divided the proletariat into rural proletarians (in Russian селские пролетарии) and urban proletarians (городские пролетарии). In any event, in colloquial contexts the word serves handily to refer to a life of rather menial trudging wherever it’s lived.
Of course Lenin had an interest in selling the idea that everyone is actually proletariat. In reality by Marx's definition, proletariat are those who don't own the means of production (and are therefore stuck in earning by selling their labour), whereas farmers at least until the NEP died, mostly owned their own farms which means they did own the means of their production, which is also why farmers, or virtually everyone in the USSR outside the cities hated the communists.
But I got your point.
Your comment is incredibly uninformed (and the third such in a row). Whole rural areas of European Russia went over to the Bolsheviks, and this has been thoroughly documented in countless diaries, letters, memoirs, and literature – it’s something that anyone familiar with, say, Volga–Kama areal studies is well aware of (just as one is well aware that, alas, many of the same rural people ecstatic at new opportunities in the wake of 1917, were shot under Stalin in 1933–1937). In spite of serfdom having been abolished under Alexander II, or having never been enforced at all in some areas, smallholders regularly found themselves falling into debt to powerful rural magnates, and exploited through those magnates’ “company stores”. The Bolsheviks’ depiction of a “rural proletariat” oppressed by a “rural bourgeoisie”, however unorthodox it might have been compared to Marx, proved easy for rural people to sympathize with.