As we all know, nose dives happen because the board runs out of power to keep you balanced. If you keep applying more weight to the front, the motor will have to compensate that with extra torque, until it runs out of it, and you nose dive. It is impossible to design a self balancing board that does not nosedive at the limit.
Also notice that nosedives have nothing to do with going a particular speed, but actually with the load on the motor. The top speed you'll be able to achieve depends os several variables, such as slope, tire pressure, rider weight, terrain, wind, rider stance, etc...
Just because you hit whatever mph under certain conditions, it does not mean that you'll be able to do it under another set of conditions.
Yes, delirium advertises it is possible to achieve 20mph. This is not a guaranty you'll be able to achieve that speed under all conditions.
As for why pushback doesn't happen right before you nosedive, the motor needs to apply extra torque to lift the front to create pushback. If the motor is almost running out of power, obviously, it cannot create pushback, it just doesn't have the spare capability to do so. That is why, if after the inital pushbak you still keep more weight on the front, the board will eventually level off, and only after that nosedive. I think that is where the "I never felt pushback before nosedive" myth comes from.
Now, what I do agree FM is very guilty of is not properly communicating this in the manual.