The suggestion is that hardening is less valuable than obscurity. As long as your choice is secret and uncommon you'll probably be safer than a well known hardened OS.
On an air-gapped machine? How would they determine the OS, so long you're careful about how the files you export are written? The article rules out TEMPEST and physical attacks.
It is stupid to think that obscuring the OS would delay them by any more than a couple of hours at best.