If they asked for a new/different/changed flight plan, then they would have to hold till the request was put through flight-service. And when in the system, ATC will clear them to go. Something as a military pilot I had to do when "command post" said new plan... and then we are put in holding by ATC until we call flight-service, put our new clearance on request, and then wait till ATC gets it and has time to give us our new clearance and let us go. (I learned my lesson, next time I was going to my first delay point in the sticks (AR track), then put in the change where we have to orbit anyway, and terminal traffic is not a problem.)
They could be training and asked to hold inbound to the Buckley VORTAC with specific legs, it may of been on their flight plan.
Traffic could be a problem, but usually nearer the terminal area, near OKC. Could be training, or a mission requirement (comm relay or whatever they do... ), or a clearance/flight plan problem.
They were high so they would not be talking to the local towers or be a problem for the Denver airport. Iron would be under Dever Center control.
https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/ifr/#highAlt
All the charts are online now.
TACAMO planes are stationed Tinker, OKC, their home.