I looked at the instructions for that cooktop, and the words don't mention hooking up a neutral to anything. I think you have found an error in their documentation. Is there a neutral wire attached to this device? Without a neutral wire from the device, 8/2 will work as you describe. If there is a neutral wire, then you need 8/3.

The instruction say "A white (neutral) wire is not needed for this unit." meaning in it is a straight 240 V load.

It is down under the "In Canada..." warning.

240 V loads do not require a neutral.

Two hot legs and a ground is all.

use 8/2 ad re-mark the white as a hot with a turn of black tape at each end, and at any other junction boxes.
Red to white, black to black. ground to ground.
At the panel land both black and white on opposite legs of the two pole breaker.
Ground to the ground bar.