He worked with a synthesizer programmer and patch developer named Dave Bristow while at Yamaha. There was a lot of mutual respect and a certain amount of mutual incomprehension, because Chowning wasn't really thinking about FM musically. Nor were the original Yamaha engineers. The predecessor to the DX series, the GS-1 and GS-2, initially had a lot of harpsichord-like sounds.
The DX7 was a hit because of the polyphony and touch sensitivity, but also because the presets - especially the synth basses and e-pianos - were so obviously musical and immediately useable.
But then you listen to the music Chowning was making, and you can already hear a lot of the tone quality. It's academic electronic music, so no bass and e-piano, but it's pretty impressive for 1977. So it's clear he was musical in his own way, but more not interested in using synths in pop than not interested in music at all.