I think it’s important to put the context what statements like these mean for most communists (this is mostly those lurking in these discussions that are not MLs themselves). The goal is not to vindicate Stalin, I think most people you left the spaces have serious problems with some of his actions. The point is a rejection of the western narrative. The history of the Soviet Union fed to us by the West is built on lies and propaganda, designed to give us a mental model of an a state that is inherently “authoritarian” and evil. By building a new mental model of Stalin and the Soviet Union, we can understand and criticize it from a Marxist lens.
As someone who used to have similar views(Ukraine, xinjiang, etc), the answer lies in Western propaganda. I think it’s because of the the reporting on these issues often includes facts that are deeply woven in to the propaganda, in a way that approaching the subjects the same way you would other news can easily lead you astray. Combine this with the fact that Western audiences are not going to be familiar with the subject matter, materialist analysis becomes actively difficult using strategies that will probably serve you well on other issues.
The way I realized my error is that I pulled a Marxist Descartes, where I did not trust any Us Media sources and rebuild my opinions from scratch using international and academic sources.