Of course they should be required to repay the money. That is ridiculous! They scammed Spotify and their listeners. Those people deserve their money back. They paid for nothing and got nothing but cheated out of their hard-earned money. The band needs to repay the money they made off this scam.
The most powerful part of music is the silence. Listen to an orchestra, and you'll find that you're most captured by the parts where there isn't a single instrument playing. Vulfpeck may have exploited a loophole in Spotify, but that isn't to say that their album of silence isn't still considered art, and isn't protected as art.
Spotify and other streaming music services cheat artists and bands, giving them only a fraction of the money that they deserve. Vulfpeck simply found a creative way to artificially inflate the number of plays that they got. There is no reason for them to return the money they earned this way.
This was a great idea by Vulfpeck. Spotify was too stupid to realize that it was a silent album before putting it on its service apparently, so they deserved what they got. Once they realized that it was silent, it seems fine for them to remove the album, but they should not be able to force Vulfpeck to repay the money.