While I also agree with the sentiment, I do think it's a little bit like saying "90% of all programming is flow control and assignments"--it hinges on wide definitions that can be contextually helpful, harmful, or both. Resolving to those chords from tones that are outside of the key? Tritone substitutions? Modulations that change what chords exactly ii, V, and i are? &c.
I think it also depends on what jazz is actually being discussed -- it applies well to most jazz standards, but a lot of modern jazz and jazz-adjacent (or perhaps just what I'm familiar with) seems to gravitate heavily towards nonfunctional harmony.
I think it also depends on what jazz is actually being discussed -- it applies well to most jazz standards, but a lot of modern jazz and jazz-adjacent (or perhaps just what I'm familiar with) seems to gravitate heavily towards nonfunctional harmony.