I guess it means in the sound scape as a whole, potentially including multiple instruments. But to keep it to a single piano, e.g. if you play B3 D4 G4, then it won't sound like a B chord (neither minor nor major) because there's no fifth. Instead, you'll hear D5 as an overtone of D4, and G4-D5 will be the lowest fifth that you hear, so it is readily interpreted as G/B. If the third were the most important, then this would have been a Bm, which is pretty clearly isn't unless you add an F# (or there is some other instrument in the mix that provides one).

At least that's how I interpreted him :-)