I don't get this part, as I always think of Bird as a "harmonic" improvisor, and of course one of the handful of the best. His "melodic" side is the amazing ability to create new, coherent melodies within the changes, but by melodic improvisor we usually speak of artists who mine the original melody for solo material rather than relying on the changes as a deep structure for new utterances, to borrow a Chomskyan concept.
But it's not an either/or dichotomy. Dexter Gordon, for instance, could be called both a melodic and a harmonic improvisor. Like Prez, he was cognizant of the lyrics and stories of the songs he played, but lots of his improvisation comes out of Hawk and Bird (and his penchant for "cute" quotes is not necessarily in the service of the original melody). I'd call Stitt, on the other hand, more purely a harmonic improvisor. I think that's why Stitt wasn't really a great fit for Miles in 1960.
I think a case can be made that Ben Webster started his career firmly in the harmonic camp and ended it firmly on the melodic side.