Did you see the rear wrap up?
The Chart said 583 max HP.
Malodave
At assumed 15% driveline loss for RWD it should have been at 597 hp at the rear tires (based on 702 hp flywheel), but that's just a rule of thumb. Considering the TRX is a full time 4WD, not sure how doing a rear dyno only test would effect driveline loss, even assuming they disconnected the front shaft.
4WD car has around 27%-30% driveline loss (Flywheel, Clutch, Gearbox, Front Diff, Front driveshafts, Propshaft, Rear diff, Rear driveshafts, Wheels), but that's on a 4WD dyno.