In response to this initial statement

Then if someone from Wisconsin accesses their content via a VPN, well, literally the point of doing that is that your traffic no longer appears to be coming from Wisconsin. So how would they know?

My argument is that sites don't have to know if you are from a jurisdiction that requires an ID, if they want to take the "leave no chance" approach of instead ensuring you're not from such a jurisdiction. They will know you're on a VPN because they can reverse lookup an IP to see that it is owned by a VPN provider (or VPS provider), and the site can choose to either block you or ask you to age verify based on what they do know instead of what they don't know.