Yeah but like... That's also true of capitalism. Grain can still paint from his desire to paint. Grain just goes and does a job instead of hunting because that's how he and his fellow humans will be able to eat later or get other services that they want/need. If Grain is good enough, Grain doesn't need to hunt at all because Grain trades art for food.

Hobbyist work exists outside of economic systems...

I guess you can argue Grain just came across the materials and the cave and didn't have to pay ... where as now you need to buy stuff to actually do the painting... But also that stuff is way nicer and made by other humans.

Also, I bet if Grain was spending all of his time painting in the cave and not helping with the hunt, his fellow caveman would tell Grain he needs to do his part if he wants to eat.