it is indeed a bit nuanced, but the 20mph limit is explicitly mentioned, with the actual language around the speed limit being:

"...whose maximum speed on a paved level surface, when powered solely by such a motor while ridden by an operator who weighs 170 pounds, is less than 20 mph."

this actually doesn't imply a motor cut-off at 20mph, more like the motor can continue supplying the amount of power that is required to go 20mph on level ground indefinitely, and if the rider is capable of adding some power, you can in fact go faster with the motor still active. however, that amount of power is really only 150-300 watts, depending on tires and drag and all that, far less than what most full weight ebikes will do. the definition is very clear that you can't have a motor pumping out 750w while you're going 25 or 30mph. it's a fairly clever definition but i have never seen any document or vehicle which actually takes into the account the nuance of that wording, in other words, that the controller allows operation up to 20mph on level ground solely from the motor, but doesn't actually cut that power off immediately at 20.1mph, which i don't see as required by that definition. there's also the obvious fact that a 750w motor can propel you UPHILL at 20mph, which i assume is allowed, but not specifically stated, nor do any ebike controllers actually care if you're going uphill or not!