I see what you mean, but I don't think you've quite put your finger on the problem. It seems to me that the later use of the term is a kind of long tail trailing from the mediaeval usage, so I don't find  it a problem talking about the former in an article about the latter, any more than I see it as a problem talking about film adaptations in articles about books. But you're right that there is something wrong with the hatnote. The problem is that by saying, "This article is about the historical Principality..." we imply strongly that there exists a later principality, which should be covered in some other article. But there isn't; there's just a tail of usage. So you're quite right that we need to change the hatnote, though not I think to "... is primarily about..." which still implies that there's some other principality. But then, do we even need the hatnote at all? We already have, as the last sentence of the introduction, "The term principality is sometimes used in a modern sense to denote all of Wales, but this has no constitutional basis." Is it too much to hope that a confused person will read that far? If it is too much to hope, then what we need to change in the hatnote is the definite article (which is what implies that there exists some other Principality of Wales). We could do something like this: "This article is about a historical entity (1216-1542). For use of the term "Principality of Wales" to describe Wales after 1542, see below." Or even: "The Principality of Wales existed from 1216 to 1542; for use of the term to describe Wales after 1542,  see below"