Interesting.  I don't think I've ever seen a piece like this scored for massive choir and drum and bugle corps.  Did you hear the reverb delay when the drum and bugle corps director gave the whistles to signal; it was about a second in duration.  When they started marching, I was sure that the reverb was going to kill the musicians as they got more and more spread out, but no, they watched the conductor/drum major, which is all you can do in a situation like that, and everyone stayed together.  My high school had a crack marching band that I played with for 4 years, so I really know what these kids were up against.  I've marched behind circus elephants (yeah, as bad as you're thinking) and I've played in a huge outdoor amphitheater, I've marched in the rain and the snow as well as blistering heat and humidity, but I never played a large indoor amphitheater and had to deal with that kind of reverb.  These kids are great, singers and musicians!  Kudos to all.