Apparently, after the creation of the USSR, Moscow spent a lot of time and effort trying to convert even tribal people who relied on seal or walrus hunting for sustenance to join the collectivization plan and believe in communism. However, in the anthropological/historiographical theories established in Marxist orthodox canon, these groups' lifestyles would be akin to "primitive communism" , while the State in "Communist" regimes was in the alleged transitional phase of Socialism. What was their rationale for imposing a regression on societies that already are living out the supposed end-goal? Did they try to look at how self-organized communities lived and extrapolate methods towards developing the ideal egalitarian society they claimed to pursue?
“Primitive communism” is not the end goal of Marxism and a transition to state socialism would be seen as a jump forward skipping multiple stages of development rather than a regression. Marx and Engels believed that human societies passed through stages marked by technological advances in the means of production and the structuring of society around newly emerging relations of production.
Communism as an end goal for human society incorporates technology and division of labor and other elements of the capitalist mode of production to create a surplus which is shared by all.
In the view of Marx and Engels, primitive societies were marked by a lack of surplus production. Without more advanced technologies, labor productivity was limited and without hierarchical organization and clear divisions of labor to create an upper class to confiscate surplus value, families and clans would simply work as much as they needed to provide for everyone and everyone would do whatever kind of work was needed.
The emergence of hierarchy eventually allowed upper classes to accumulate wealth and their power allowed them to coerce others into working more in order to further build their wealth and power. As labor tasks became more specialized, clear divisions of labor were implemented to increase efficiency. (As an interesting side note, Engels believed that patriarchy was the first form of social hierarchy to emerge in early societies.)
This comment is my own interpretation based on Engels’ “The Origin of the Family, Private Property, and the State” and it only covers the aspect of the question that deals with the view of primitive societies in communist orthodoxy. It does not attempt to address problems with Marx and Engels’ conceptualization but merely attempts to reflect what they thought. Hopefully someone else can provide some insights on the flaws in their theory as well as the specifics of Soviet policy on indigenous peoples and its contemporary justification.