I can think of three scenarios that explain the need to run the ICE.

1. As has been mentioned before, at a certain speed the ICE has to spin to protect the motor generator from overspeeding. That speed appears to be approx. 42 mph.

2. (conjecture based on personal observation) At some point the motor generator will spin up the ICE to consume excess battery power. Since the computer works to prevent the battery from being over or under charged, if the charge level gets too high it has to have a way to consume some of that energy without increasing the vehicle speed. This appears to be what the engineers chose to do. You can see the energy flow arrows reflecting this at some point.

3. Because on downhill stretches the battery is accepting a charge at a higher rate than at other times, enough heat may be generated as to require the battery AC to be turned on. This only works when the ICE is spinning.

Note, in all these cases, while the ICE is spinning, it's consuming very little fuel. You can try the "L" mode on the gear selector and you will see that once the battery is charged the system will gladly spin the engine up to 4000 RPM to help brake your descent.

By the way, coasting downhill without the clutch and transmission engaged is illegal in many jurisdictions.