I don't think this is a problem with GrapheneOS or something they can change, but rather Google would have to add GrapheneOS to their list of "officially approved" OS. Someone can correct me if I'm wrong about this. But I think it's the similar reason Netflix and Hulu both can't be installed on GrapheneOS.
EDIT:
Also I think the app developer could fix it too by not requiring the device to pass the full Safety Net check ( I think that's why it says it won't work on the device?)