I take it that with the choir being more in-tune, that the notes reinforced each other, so they died out slower? I think that makes some sense physically. I think that should hold even with a digital reverb unit. I have an old Yamaha MIDI module that supports a variety of tunings (including harmonic, for a specific key), I will try to drag it out and see if I can replicate that....
As a real-life example : working with a chamber choir, we tuned to the (digital) piano CGEC, sustained the chord then cut it off. For 15 seconds the echo persisted in the space. Then again, sans piano, tuning harmonically to each other. 34 seconds.