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).
Allowing that I'm simply not sophisticated enough to understand this, I think he's overthinking things. :-)
I mean, B3 D4 G4 in isolation probably does sound like G/B to western ears, and maybe the overtone explanation is acoustically relevant, but it seems like there are not too many other things that those notes could be, and the spelling clearly spells a Gmaj triad.
At least that's how I interpreted him :-)