Reinhard: Leeneia wrote that the book she was thinking of also included the tune later used for Good King Wenceslas, not that Personent Hodie were that tune.

Having re-read Leeneia's post, I think it can be interpreted either way, since there are pronouns with unclear references, but since your interpretation gives something more factually correct, I will agree that you read it more accurately than I did. :-)

I'll still (mostly) stand by my second point, though. In my experience, when songs from the Piae Cantiones are filed by age, they tend to be filed among Renaissance, not Medieval, works. Sure, they're Latin, and sure, some are pre-1500, but it's not really possible to prove it in most cases, so people go with the safe date. :-) And while the Reformation brought in a lot of vernacular hymns, not everyone reformed all at once. :-)