How could trespassing laws possibly apply? It’s not the airlines property just their vehicle parked on the government’s property

By that logic a stranger could get into my car if it is parked in the street, since it is a private vehicle parked on the governments property. This seems counterintuitive.

But the vehicle is not parked in a public space it's parked on private property that is owned by the government. If someone is in a vehicle on someones property then it's the property owner not the vehicle owner were trespassing laws apply to.

The Airline is as much a guest in an airport as the passenger is.

This is not true. In the US, trespass in a vehicle is often criminalized regardless of where that vehicle sits.

Vehicles are typically included as property under most trespass laws