What about on USB, when MCC playback is instant and doesn't require a pause?
I did a quick test to compare how many keys I can press in 15 seconds.
Pressing a single key over and over I got 85 presses.
Cycling through L000 0L00 00L0 and 000L I got 89.
On a standard keyboard typing "asdf" (same finger pattern as on the Twiddler) I got 112 (134 ms/key).
So that's 25% more keys per second when I don't have to release a key before pressing the next. I can't think of a way to easily measure how close together in time I press the keys of a chord on the Twiddler, but it seems significantly shorter than 134 ms... probably under 50 ms. So I'm still under the impression that I'd type a bit faster if I didn't need to release keys on time, and that it's practical to discriminate chords in this way.
I've also done some experiments with two-key chords using Emacs key-chord-mode and a standard keyboard. In normal typing I definitely hit keys closer together in time than 134 ms, evidently even as low as 25 ms sometimes (assuming the timing is accurate in Emacs). So the proposed threshold would have to at about 25 ms (for me personally). And I'm not 100% consistent at hitting the chord versus the individual keys. I'm pretty sure you could train yourself to be better at that though, and I'm more reliable when the chord is one-handed anyway.