That's one method they use, but "just cluster" is doing a lot of heavy lifting in that sentence. It's why Erik Bernhardsson came up with the Approximate Nearest Neighbors Oh Yeah algorithm (or ANNOY for short)
> We use it at Spotify for music recommendations. After running matrix factorization algorithms, every user/item can be represented as a vector in f-dimensional space. This library helps us search for similar users/items. We have many millions of tracks in a high-dimensional space, so memory usage is a prime concern.
Yeah, I am obviously not a data scientist.
I guess where I was getting at is they do not technically even need to know genres to recommend songs. In practice though, they probably have to know them anyway for playlists, but I assume they can have the song owners provide that when the songs are uploaded, and artists specify it when they create their profile.