I think it's the only option they have, the other would be banning Kairos but since he's almost essential to any competative Chaos Daemons army because of his Warlord trait and the 1 re-roll that's not going to happen. Imo they made a good call banning the Grimoire as it makes Screamer Star unplayable without hindering other possible builds.
If it were only that simple with the others a simple FaQ would clear the whole balance issue.
You could maybe ban the grimnoir from armies that also contain Kairos? Slightly less draconian than banning Deamon armies that don't contain him from being restricted. I hope this is not a trend as I would dislike this artificial handcap to be imposed in the UK.