My reading of the NTSB report is that the crew were in fact, exceeding limitations. On all of the takeoffs from the crash site helipad, they were using 2.5 minute OEI (and co-incidentally, topping) N1 limits (the rotor was drooping at a constant N1, which would only happen when the compressor can't turn any faster.
Takeoffs at lower altitudes did not result in rotor RPM droop.
If that's not exceeding a limit, I don't know what it's called.