Yes, my company launched an NFT app that is not on any blockchain (although we're looking into how you can export your NFT to a blockchain of your choice if you want). An artist can certainly sell their art in our app and on ethereum, say, just as an app can be on iOS and Android, or a stock can be traded on multiple markets. We have had people try to upload copies of bored apes and so on and we reject those, but if it's the artist, that's fine.