So he explains OFDM in a way that implicitly does Amplitude shift keying.

I guess if you want to use different modulations you treat the complex number corresponding to the subcarrier as an IQ point in quadrature. So you take the same symbols, but read them off in the frequency domain instead of the time domain.

And I guess this works out quite equivalently to normally modulating these symbols at properly offset frequencies (just by the superposition principle)