The question is ambiguous, but the answer is probably supposed to be "decreasing".
The explanation provided is sort of misleading if taken to the word. The flat-rated portion of the takeoff rating curve must also account for the thrust lapsing of the clamped-throttle engine as the airplane climbs out on takeoff. The engine control controls the fixed-throttle lapse as altitude increases - the N1 or EPR decrease with increasing airspeed as the airplane goes down the runway, then will increase as the airplanes climbs out. In the old days, the engine controls typically maintained a fixed N2 with fixed throttle, but design of lapse rates has varied over the years. The speed or temperature limits can be reached at any point in the takeoff up to some assumed point above the airfield - typically 4,000 ft. but sometimes less. The takeoff corner-point temperature is typically ISA+15C (i.e. 30C at sea-level) but can vary.
I would disagree that the flat-rated portion of the curve is up against a pressure limit, even when considering the lapsing. The flat is usually chosen to provide the full-rated thrust desired by the airframer, if possible. There is a big set of trades that come into play - and engine life/durability is one of the keys. Only when you get to the higher takeoff altitudes, with lapsing considered, do you really start to encounter limits such as rotor speeds.