He knew, he just didn't give a shit until Rudolph happened to be useful. It's a capitalist moral sorry, showing that you will get things and be treated well only if you are useful to the rich overlords.
Why the fuck does the song assume we know Dasher, Dancer, Prancer, Vixen, Comet, Cupid, Donner, and Blitzen, but it has to ask if we recall Rudolph, despite him being "the most famous" reindeer of all?
Back when it came out, it was a tongue-in-cheek joke. “SURELY you must remember the most FAMOUS reindeer that we certainly didn’t just make up, right?!”