Generally speaking the throttle will have 3 wires: ground, 5v and signal. where the signal wires varies anywhere from 0 - 5v. Going by the pictures on Amazon, your controller seems to be built into the battery, or in the housing which holds the battery.
To find the controller look for the wires coming from the PAS or pedal sensor, to see where that goes. If your controller has a spare 3 pin port, then this could well be suplied for markets where throttles are allowed. Controllers tend to be generic items and so it may have a throttle input by default, even if its not used.
That said, i was thinking this could make an interesting project. In theory it ought to posible take the 0 - 5v signl from a throttle and convert it to the pulse signal expected from a pedelec senor. An Arduino could in theory do the conversion to a pulse where the timing was based on the speed required.
It might even form the basis for an after-market solution to convert pedelecs to throttle.
If you do find the controller, post a pic.