As is made very clear by at least one of the books in your list, Daniel Leech-Wilkinson's
The Modern Invention of Medieval Music.
Highly recommended (and not only because my name's on the acknowledgments page ... though that did make me very happy
).
Indeed - an extraordinary book. Wouldn't go down so well with HIP positivists, though. His chapter in Knighton and Fallows is also well worth reading as well.
Speaking of which - we've never had a wider HIP thread on here (about the whole business of historically-informed performance, its positives and negatives, and so on), though the subject has been touched upon briefly in other threads. What do you think?