As a former hotboxing teenager I would argue that hot boxing a car doesn't do anything to increase THC plasma concentration, but it is a great way to restrict oxygen
As someone who for much of his youth socialized with pot smokers without being one himself, I've had enough "contact highs" just from sharing enclosed spaces with these people to know the second-hand effects are present (though weak).
If hot boxing a car over an hour consumes enough oxygen to matter it wouldn't be possible to sleep in a car without running out of oxygen. A couple blunts doesn't consume that much.
My point is that the oxygen consumed over the duration of hotboxing a car is so trivial compared to spending hours sleeping in a car that if the former has any effect the latter should too.
Automobile passenger compartments are vented, otherwise there'd be problematic pressure spikes on door/hatch closures. You're not getting deprived of oxygen sleeping in a vehicle.