If the carpool lane’s purpose is to reduce the amount of cars on the road his rule actually makes a little sense. (only a little)
It will only help to reduce the amount of cars on the road if the alternative to carpooling would be driving separately. So you need to get everyone to prove that they would have otherwise traveled in separate cars. Dropping someone off somewhere wouldn’t qualify for that, taking a taxi with one passenger wouldn’t either and driving someone without a drivers license around would also not pass the test.
It does get a little fuzzy when you drop your own child off at school together with the neighbor kid, they both don’t have a license but if the neighbor drove her own kid the amount of cars would be doubled.
All in all, I think the 2 people (regardless of license status) rule seems the most simple, straightforward and actually enshrined in the law of the two choices
edit: @Celandine_Crane worded it better then I did.