Yes, it's true: with PWM, once you have 100% signal with no interruptions, that's the absolute max an ESC can give you (as it no longer chops up the batteries supplied power but let's it run freely to the motor). Absolutely zero magic ways to go beyond that. Zero. (Remember, we're not getting into timing here). However - at least in theory - it could still be possible (yet very, very unlikely) for an ESC to behave the way described: if with calibration (incl. LED indicating 100%) etc. you wouldn't actually get to the real 100% PWM signal but something less... then the extra throttle signal might get you to the real 100%. But in my view that would be a defective ESC (or at least shitty firmware). So... sorry, I don't buy it either.