This is something I've never understood. Why do people think the terms "socialism" and "communism" are interchangeable?

Communism involves complete government control of the means of production. Socialism, on the other hand, is far more flexible, and generally calls for government regulation of an industry rather than complete government control. One such example of a socialist idea would be a federal organization that inspects the meat we eat (like the FDA), so we don't end up eating rat droppings with our canned tuna.

Put simply, socialism favors the worker and advocates for government involvement in industry, but does not call for complete government control. The USSR may have had socialism in its name, but I can assure you, the totalitarian society in which Stalin starved the populace is not socialism. That was communism, and a damn unfriendly interpretation of Karl Marx's ideals (to clarify, Marx never actually laid any groundwork as to how a Communist government would be structured. He had lofty ideas, but didn't put down any hypothetical political framework, which is probably part of the reason every example of a Communist society we've seen irl has been such a mess).

Communism and socialism share a few similar base ideas, but they are not the same. Hence, the confusion pointed towards you calling Stalin a socialist. It's like calling a chicken sandwich a cheeseburger. They're both a piece of meat between bread and maybe a few other toppings, but they are not the same thing.