THD is measured on one single frequency (tone) at once, IMD with two tones.
A classic IMD measurement is calculated only for one pair of tones and not for all other. Wouldn’t be it a huge oversimplification? I’m afraid a testing of audio equipment will be very hard on a real life signals (music, speech).
Its only an oversimplification if you expect non-linearity to be strongly frequency dependent.
IMD 19-20 kHz, spurious tone at 1 kHz -100 dB.IMD 19.1-20.1 kHz, spurious tone at 1 kHz too - 100dB.IMD with 4 mentioned tones would be twice. -94 dB (?)
Unfortunately the "standards" are far too loose and allow for inferior equipment makers to make dubious claims. For example I feel that amplifier makers should be forced to run full 20-20k white noise before publishing power handling/thd levels rather than "100 watts x 6......" (but only if you run one channel at once at 1khz)