Pretty sure it's an autism thing or something associated with it but I always need 300% confirmation, but is not being able to remember people's faces an autism thing? like they're just blank slates in my memories that are producing noiseless words magically.
That depends on what you mean by blank slate. If you can’t close your eyes and imagine what they look like by “seeing” them, but you can remember traits that make up a person and you recognize them when you see them, it is aphantasia and it is not an autistic thing.
If you cannot tell similar faces apart from each other, or you cannot recognize someone if, say, they change hats, or put glasses on or take them off, dye their hair, wear different makeup, etc., that is prosopagnosia or “face blindness” and while it isn’t specifically an autism thing it is something that is much more common in autistics than in the general population.