A quick review of published information reveals this claim as false. A typical airliner FMS feeds information to IFE via gateway devices. The integration is intended to be one-way. Airgapped they are not.

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, particularly with regard to integration. There was a good talk at DEFCON 22 by Phil Polstra on the matter.