Yes, I was thinking the same thing; somehow, the media must create localized anaerobic spots for denitrification to occur.
However, what would be the purpose (if anaerobic bacteria truly are present)? Denitrification would take the nitrates that are produced by the aerobic bacteria, and convert back to nitrites...which are then converted back to nitrates by the aerobic bacteria.
On top of this, the aerobic bacteria would be much in excess compared to the anaerobic bacteria, meaning nitrification would be very favoured.
I think, and I'm not an expert by any means, the actual process in denitrification involves bacteria reducing nitrates to nitrites, and then reducing that to two things; nitrogen gas and ammonia. The part that's converted back to ammonia will get used again by the bb, but the part turned into nitrogen gas is not usable and is lost to the atmosphere.