Utah argues that websites can "age assure VPN-using Utahns, [so] any decision by Aylo to instead age-verify all users would be a voluntary business choice, not an effect forced by Utah law" because regulating non-Utah users on non-Utah sites is not something the Utah law can constitutionally do under the Dormant Commerce Clause.

So in theory sites could block all VPN users (if you can somehow "perfectly (1) detect VPN/proxy users"), but if the law necessitates that nationally/globally then it's unconstitutional anyway. It's the age assuring of VPN-users specifically from Utah part that Utah are relying on being possible for the law the be constitutional.

I have no problem connecting via a VPN running on my VPS, so it's not all VPN users.

A VPN/proxy could exist on any given IP address at any time, which the California law recognizes but the Utah law does not.