Interestingly, the post says the model is Attribution 4.0, which by itself does not require any payment.
From what I understood, if you do not pay, it is technically a CC BY-NC, and if you pay, then it is a CC BY (Still requiring attribution).
Though OP seems not to respond people on Itch anymore.
He did, although, say this in specific:
“you just buy them and then you can use them in commercial projects”
So I assume that’s it.