The "maximum speed" that ThrustHP gives is not based on the airframe. Since there's no airframe specified, how can it calculate the maximum speed?? That speed is the "zero thrust" speed of the propeller. This means, that at this speed, the forward motion of the aircraft will cause the propeller blade to have a zero relative angle of attack, aka zero pitch. This is also the reason why an engine unloads with flight, since the AoA (relative, the absolute reaians the same, as long as the prop is rigid...) of the blade reduces as the speed goes up. Reduced AoA means less lift and consequently, less drag, so the prop "eats" less power to rotate at a given RPM. Since the engine power doesn't change, it revs up. This is explained by simple vector physics.
But there's a serious flaw with the ThrustHP calculation, and that is that it assumes that the prop will generate 0 lift, at 0 AoA. This is only true for symmetrical (non-cambered) airfoils, what no prop (at least that I know of) uses. The kind of airfoil that most props use is cambered ones, resembling tha famous Clark-Y. And this kind of airfoil generates 0 lift at a negative AoA, so the speed given by ThrustHP is lower than it should be. And, as each brand of prop uses a different airfoil, it's very difficult to have accurate results using generic data.