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.
Such analytical delusions are the first step on the road to failing to adequately mitigate threats. As practiced by “it can’t happen here” school of fucking up.
Fortunately, it seems far more likely that aircraft system designers do not rely on any such assumption, and practice defence in depth. There was a good talk at DEFCON 22 by Phil Polstra on the matter.