Whether the terrorists who piloted those aircraft knew what the barber pole was, or what the significance is of Vmo/Mmo, is frankly irrelevant, because they did not care. They were about to plow into buildings - something I think we can all agree that anyone with a pilot's licence usually tries not to do, and is "trained not to do".
So even if their sim instructors had explained every detail of the flight testing at and beyond Vmo conducted by Boeing, had explained every element of the flight test and certification basis for the definition of Vmo at low altitude, it still would not have mattered. they wanted to fly into those buildings, as fast as they could, to cause the most damage they could.
There is nothing aerodynamically special about Vmo which makes it a cliff-edge past which instant disaster threatens. I've said this before, but every aircraft certified to Part 25 is tested way above Vmo and Mmo, and usually the critical case for handling is Mmo, which they were nowhere near at low altitude.
Flight well above Vmo is dangerous - even very dangerous - because you are compromising all of the dsafety margins built in for cert. But unless you manoeuvre violently, or encounter a severe gust, or heavy turbulence, then there's no real reason the aircraft will suddenly fall apart. It's a stupid thing to do, because you are taking risks that are not required. But they had no intention of completing their flights anyway. Even if they had done it and then lost control at the late stages of the attack (and who's to say the attack on the Pentagon at least didn't?) then the two WTC aircraft would still have come down in downtown Manhattan and caused significant property damage and loss of life. I'm sure the terrorists would have considered that an acceptable result.