I said "more medievally" - I never said exclusively medievally. But the 5th ed Faeries supplement requires the reader to work pretty hard - between all the stuff about faeries being embodied stories and the Babylonian Goddess who kicks off the bestiary - to find anything which is based in the European middle ages.
For example, at the moment I'm putting together a story for my players based on legends of Thirwall Castle and the borderlands tradition of faerie ointments and penalties for thieving such. These are, of course, modern recorded stories, and in the recorded tellings there are plenty of elements which seem post-medieval (e.g. the modern folk tradition's motif of "returned from Crusade" as a convenient hand-wave for anything alien), and it is an interesting in trying to dial back to a time which we have few references from and a skewed understanding of. But the stories are, at least, consistent in feel with the texts we do have from the high middle ages (I'm thinking of Walter Map and Gerald of Wales, for example), and both these modern folk traditions and the medieval reference points are absolutely viable under the 4th ed rules. Trying to make them work with the 5th ed supplement is just pointless - how is any of this consistent with, or improved by focusing on, ideas like cognisance? It isn't. Whenever I try to look at a medieval or later faeries story as a source for an Ars Magica plot, the 5th ed supplement is both irrelevant and unhelpful.
But you're right. It is its own thing. It is a modern fantasy creation which is now part of cannon, and so, sure, my desire to have more medieval faeries is out of step with canonical Ars.