To answer your question, they pocket it back by paying it to themselves. The artist is only formally an external ”artist”, but in this case he’s essentially one of their own. They curate him, let him on all the most promoted playlists and keep an appearance as if they’re paying it to some artist just like they do to anyone else. That artist gets all this firepower by agreeing to keep less of the cut while paying more of it back to Spotify in some other ways and it is all kept under the radar. That’s what I suppose.
Just a guess but maybe not all deals between Spotify and artists are the same (% royalties paid)?
If that was the case and Spotify music picking algorithms chose artists with lower % royalty, Spotify would save money.
My understanding is that that's not really how Spotify's financials work, in that how much will be paid as royalties _in general_ to everyone is calculated first as the "royalty pool" (something like 66% of revenue), and then how that money is split and to whom is calculated secondarily (using an algorithm called "streamshare").
So in this case, its affecting the "streamshare" calculation, but not the "royalty pool" one, and so not affecting how much money is in Spotify's pocket.