That’s an interesting comparison— the quotation from Slim is from soon after the war, though, so it’s really a contemporary view of the conflict.
I think the German supermen/Slavic hordes interpretation was very much a result of the postwar situation: a defeated Germany in which many officers returned to society in the West, wrote memoirs, and developed the “not my fault - Hitler was crazy - how can you defeat millions of Rooskies?!” meme at a time when those same Rooskies were our geopolitical enemies, their archives were closed, and anything they published was in an atmosphere of political repression and propaganda such that you really couldn’t credit it. Time has been the best antidote: the Russians archives were opened, the postwar German memoirists died, and now we can have some sort of more informed reckoning.