I'm getting that same cooktop. My cabinets are custom and the KD specified a 24" deep cabinet.

That seems correct to me too. The cooktop specs are under 20'' for the unit and 2'' clearance in back. I'd like the extra 2'' from the cooktop cab for the island width.

Remember that the interior of a standard 24" deep base cabinet will be 21"- 22". That only leaves 1" max accessible at the front and back of the cooktop from below. Induction cooktops need more ventilation than do standard electric cooktops. Perhaps that is the concern, as well as the very tight fit into the cabinet space. What exactly does the manufacturer's specs say about clearances required?

I was hoping you would see this, LWO.
See pages 44-46 in the link below. I read them as specifying 20 1/4'' cutout.

Here is a link that might be useful: Miele manual

The installation instructions show that the cutout is 19 11/16" front to back and the instructions say: "Remember to maintain a minimum distance of 2 inches between the rear wall and the right or left side."

For ventilation it says to leave 4 1/2" open below the cooktop. The part that is confusing to me is it also shows a 32" cutout at the rear wall of the cabinet directly below the cooktop for ventilation.

My KD is very experienced and I know she went over the installation instructions of all my appliances very carefully.

Here is a link that might be useful: Installation instructions