They could, for the sake of a ridiculous but clear example, have a display hooked up to the avionics and a camera hooked up to a separate computer which reads the values.

There are various ways of connecting systems while physically guaranteeing one way data flow—a fiber optic link with the transmitter removed from one end and the receiver removed from the other is basically a less silly “camera pointed at a display” and used in the real world.

You could argue the exact semantics of “air gapped”, but for the discussion here that’s accomplishing the same thing. The fact that the passenger network has some visibility into the avionics network is not, in and of itself, any indication of an issue.