yes Nicky is doing pretty good, even with his "sportbike" style ... but I had this exact discution with pro pilotes coming to supermoto once in a while, they all say that it's not always the best position (but there are so used to it they keep it this way).
Just look at the others (Biaggi, Rossi... Dymond ...etc) that ride supermoto sometimes...
The biggest difference between a sportbike and a supermoto is the way it brakes: Supermoto allows you to brake a lot closer to the curse, on many narrow curves you just turn like on dirt : hard brake, fast turn and full gazzz ... as with a sportbike (heavier and much more powerfull) you have to cool things up if you don't want to do a 360° !
But in a way your instructor has a good point, exactly like in MX racing your feet HAVE to be on both pegs AS SOON HAS POSSIBLE !!!
Bracking: you want to keep both feet on the peg as long as posible to control the bike more accuratly.
Gazz: well, more power you transmit verticaly to the bike, more grip you'll have on the track, so as soon as you can, you put you feet back on the peg and put pressure on both !