His actual impulse response test is done with 55us pulse. That would correspond to about 19kHz frequency, thus it does not show effects from overall group delay down to 20 Hz. Nevertheless, what you see is a big positive pulse followed by a smallish overshoot, about 15% at most, but which lasts a bit longer then the initial impulse. Thus from one cycle input you are getting two out, but the second is greatly reduced. He claims that the length of the tail on the impulse response corresponds to 12kHz resonant frequency.
I wonder what this would look like with a 1ms pulse. Probably more complex, like I get from my Acoustat.
Then he shows calculated step response. If reproduced accurately, that wouldn't even be a half cycle. But it does show initial decay in about 1ms, overshoot, and some LF resonance after that (drumhead?).
He shows pretty nice squarewaves also, with 12kHz ringing.