At its basic level,
Bach's Art of the Fugue is a fairly academic work, a set of exercises, so to speak, demonstrating the various contrapuntal ways of setting a single subject.
Sébastien Guillot's harpsichord reading of it doesn't do much to contradict that notion. His playing is dry and almost invariably the same tempo throughout. It's not too slow or too fast, it's just the same nearly the whole way through, causing the aural equivalent of the eyes glazing over. Fugue 13a is little slower than the others, but not much.
Guillot tries to make the opening of each fugue a profound statement, but as soon as other voices start coming in, it gets hard to follow just one voice. It's not just a case of faster notes coming into play and giving the false impression that the music is speeding up and running together. Granted, the harpsichord doesn't have the same expressive capabilities as the piano, and the one
Guillot uses is not bad sounding at all. However, harpsichord music can still be shaped with changes in tempo. Adding just a breath before a new statement of the subject or a minute ritard at the end of a statement would help delineate voices.
Guillot does some of this in both parts of the 13th Fugue and in the final Fugue 19. Otherwise, though, ritards at the ends of fugues seem like afterthoughts. Given such a monochromatic performance, it's no wonder other musicians look for other instumentations that will allow them to make the voices in these fugues more distinct and the counterpoint less cerebral.