Let me rephrase it: "invalid states unrepresentable" should hold true, but the hard part is that these invalid states should can change. Your model should be flexible enough to allow changing what constitutes invalid states.

Any invalid state that cannot ever be a valid state (Not just that it's not valid today - you know it can't be valid tomorrow either) should be unrepresentable. A hard requirement.

Anything that is invalid today but could be valid tomorrow (One-off change, requirement change) should not be.

This makes sense to me and forces you to think up front about what "invalid" means. There are definitely hard errors you should make unrepresentable, even in domain models.