The question is, who has the right to ask you for proof of purchase at all? Assets creator? The user of DAZ studio is not in any contractual relationship with the creators of the assets if the assets are sold in the Daz store.2D guy got asked to show proof of purchase and when he didn't his Patreon got pulled. My theory is someone snitched. So yeah he probably did fuck up, still got caught out.
This means that the creator of the product would have to ask Daz 3D to contact Patreon, stating that the developer MAYBE uses a product without a license? That sounds pretty complicated.