Gardiner was addressing an issue irrelevant to mediæval music. His thing is the Baroque, and he was trying to correct a long slide to slower tempi when playing Baroque music. There have been other times and places where the same process has happened: the most extreme is how Gagaku music developed in Japan, where what we now hear as the melody was originally the gracenotes in the Chinese originals.

But mediæval European music didn't go through centuries of being played progressively wronger. It went through centuries of not being played at all. Most of what is now known and played was never published until our lifetimes and is only known from one-off copies in manuscripts. There never were any misconceptions about its tempo.

Until very recently. Cait Webb described one school of performance as "drums and fun", imposing compulsive metric regularity and folkish jollity on the music for modern marketing reasons. The absolute pits for that, in my experience, is John Renbourn's atrocious rendition of Machaut's "Douce Dame Jolie", like a mashup of Singing Together and the Clancy Brothers. No performer with a clue would now do anything like that.