Graphene prevents the provider from being able to prevent you from turning on your hotspot, by not having that carrier-specific functionality implemented, but nothing prevents your carrier from seeing packets and detecting that a different device is using the network (and then blocking it to prevent you from doing it.)
A way to prevent your provider from detecting that you are using a hotspot would be by making everything go through a vpn, but that's not going to work if your other device doesn't support running a vpn tunnel in the first place. Making hotspot go through the phone's vpn is unlikely to ever be implemented because it's better for the other device to run its own, but hopefully this can be reconsidered given that this isn't always possible.