This is news to me, Doc. Got a cite?
Stalin executed two types of people: Kulaks and "intellectuals." Of the latter, one could be "in favor" one day, and "out of favor" the next. (The Kulak seems never to have been in favor....) Stalin resorted to a good purge every now and then -- terror being one of his tools of totalist control -- and I feel reasonably sure that whether someone was a Darwinist or not was not a selection criterion. Stalin was glad to execute victims of either persuasion. He executed Ervin Bauer in 1947 in a "routine" purge. Bauer was not a Darwinist (as far as i can tell).
As to whether Stalin was a "true Marxist," my impression is he served nothing -- no one and no idea -- except his own lust for power and the idea of his de facto "divinity."
Check out the Affair Lysenko. This is well known among biologists. Of course, the removal of Darwin from the biology and thus agriculture fields did help the Soviets set a record for crop failures.