since the early 90s, the USAF Fighter Weapons School have encouraged and included US Army and USMC attack helicopter assets into the various Red Flags.
The thing is, there's "Red Flag" the high-end full-war exercise flown in the large northern part of the Nellis ranges, and there's "Red Flag" the CAS exercise laid on in an adjoining (small) southern area to enable aircrew without the requisite experience for the main scenario to take part (and to allow any slower aircraft a chance to get to and from area in time for the debrief). The chances are that the attack helos referred to would have been taking part in the latter, which is nowhere near as sexy as the former. You can get equivalent CAS training somewhere else at a fraction of the cost (or on Ex Green Flag, which is probably the best CAS ex out there).