But
Hasn't it already been known that actual air gaps aren't secure either, because the little computers inside USB memory sticks can be taken over by hackers?
Of course, that isn't the only threat. Even if the medium used to transfer program files to an isolated computer is secure, the files, ultimately obtained from the Internet, can, of course, have been corrupted with malware. So going to the trouble of building a "secure" computer that has to be fed data on punched cards, say, is a waste of effort.
So what should India be doing, then?
For starters, build their own USB sticks which have validated firmware which cannot be updated externally. That solves one problem.
Next: have an air gap around the facilities that develop software for their power generation systems. That way you avoid supply-chain attacks.
And then you use sneakernet to get the software from the developer to the power station.
The fact that employees will not be able to browse the Internet on their lunch breaks may be viewed as unreasonable hardship. But as long as their work computers don't have USB ports available, they could still browse the Internet on their own smartphones.
Oh, wait. There's this thing called the COVID-19 pandemic, and so everyone is working from home.
In that case, you will have to settle for a "virtual air gap", insecure though it may be. However, a more secure virtual air gap is possible. Basically, the work-from-home computer connects to the server through a VPN, but in addition, neither the work from home computer (supplied by the employer, not the employee's home computer) can't connect to the Internet in any other way, it can just use the VPN to go to the office server.