The mono tracks are very often down mixes if the 5.1 tracks.
I can absolutely believe that of the second and third seasons. But many episodes in the first sound at least like what is on the laserdiscs. The Menagerie for one. However, they list these as the "original mono broadcast track," which for the most part, is simply false.
Like many fans of my generation, I audio taped the episodes in the 70's and listened to them over and over. I'm fixated on the sound, so a lot of this stuff jumps out at me and feels really wrong. So other fans' mileage my vary.