Any system accessible with a firewall exception is not "air-gapped" by definition.
A level below that is diode networks, which are not air-gapped but provide much stronger system isolation than anything that is accessible with a "firewall exception".
Far below either of these is vanilla network isolation, which is what you seem to be talking about.
I completely agree. Maybe I should have put "air-gapped" in quotes.
The most interesting subset of these systems are high-assurance bi-directional data paths between independent peers that are quasi-realtime. Both parties are simultaneously worried about infiltration and exfiltration. While obviously a misnomer, many people still call them diodes...
The entire domain is fascinating and less developed than you would think.
It is a genuinely fun project for someone with sufficiently sophisticated skill but I suspect there is relatively little money in it, which colors the opportunity and outcomes.
The absence of clear commercial opportunity gives the domain a weird dynamic.