>designed to accommodate two passengers, including the pilot
That's one passenger.
That's one passenger.
It's a "2-seater" because it would be confusing to count the driver's seat as a passenger seat.
If I tell you I left my sunglasses "on the passenger seat", I hope you don't respond, "Well? Which one?"
My point is the number of people qualified to operate a vehicle doesn’t change whether the operator is a passenger of the vehicle. I personally know more folks who can fly a small plane than drive a semi-truck.