GrapheneOS team has a pretty level take on the issue in the original thread:

This is fearmongering based on customer support making ridiculous claims to someone. There’s nothing illegal about using GrapheneOS and the customer support is nearly certainly making it all up to get the person to go away so the ticket can be considered closed. It’s unlikely the company has done anything to specifically detect GrapheneOS or ban using it. It’s far more likely they detect not using a Google Mobile Services operating system without modifications. This customer support person went on a power trip to scare someone and get the ticket closed.