Agreed on the Roberts guy being full of shit. But there is this passage in the wired piece:

"A connection between the avionics system and the IFE does exist. But there’s a caveat.

Soucie and Lemme say the connection allows for one-way data communication only. The systems are connected through an ARINC 429 data bus that feeds information from the avionics to the IFE about the plane’s latitude, longitude and speed. The IFE uses this to populate the animated map passenger’s can use to track the plane’s movement.

“On every airplane it’s done a little differently and is done in a proprietary way,” Lemme says. But in each case, the ARINC 429 is an output-only hub that allows data to flow out from the avionics system but not back to it, he says. To talk back would require a second input bus. “I can’t think of why there would ever be an interface like this. If it’s out there, I haven’t heard of it.”

This would seem to be what Boeing was describing in its statement when it said that although inflight systems “receive position data and have communication links” to other systems on the plane, they are “isolated” from systems that perform critical functions."

So there is a link.

An airgap simply means this: the two systems are not connected. Not in any way, no physical connection exists between the two and any output from the one goes through an optical bridge into the other.

The only way air-gapped systems are possibly connected is via power rails but presumably those do not carry data nor do they have the possibility to do so.

https://en.wikipedia.org/wiki/ARINC_429

Suggests it is an electrical bus, not optically insulated.