I checked it too, and it seems you're right, there's no clear rule about VPN usage.

But if you read this, restricted countries, it seems clear now that you can't bypass if you belong to this country.

When a casino restricts certain countries, it usually means residents from those countries aren’t allowed to gamble there legally.  So, using a VPN to bypass this restriction technically violates the restriction policy because it enables access from a prohibited location.