I guess there are probably real systems referred to as "air-gapped" that don't have power isolation they could be addressing, but it still feels a little disingenuous.
Security measures (should be) a line item on a budget with a total. The details are something that might be reviewed by an expert in the field with the necessary clearance. Secrecy also has a cost (security by obscurity isn't a design feature, but security //plus// obscurity might have a worthwhile value).
I think at a site level a sufficiently sized dynamo, maybe per section, could be utilized as both an initial gaping measure and as a mux for different power sources (line, stopgap, and local generation). For high value sections one or more full re-regulation stages would help. (Off the shelf, a UPS that always AC > DC > AC converts comes to mind.)
Finally, at an OS level, it seems that the highest value targets should probably examine a feature similar to the 'constant time' style cryptographic / security response paths better algorithms and designs have; in this case always running the system within a constant performance (and power use) envelope. I had not previously considered that such ultra-high security environments might be harmed by /efficiency/ as a means of leaking data.
Whether your threat model actually needs to protect against these is an entire other question.
There is also a minimum distance between the computers that must be adhered, in fact there is a minimum distance and shielding requirements for the in wall cabling between different networks including power.
And my guess is that since 2009ish the regulation have only become more strict.
A room within a room with non-parallel walls for acoustic isolation over a wide frequency range. The inner room is hanging from a spring suspension system. A mesh layer for electromagnetic shielding. A power isolation system doing some elaborate AC-DC-AC conversion (mechanically and/or electrically). Also, separate air handling systems to avoid connected duct work.
And of course, there could be significant buffering and filtering via mass loading, shock absorbers, capacitor banks, chilled water tanks, etc.
For critical applications, the system should be launched far enough from the edge of all observable matter, that it is causality gapped to the heat death of the universe.