Sounds like the AI slop mixed up a snippet from the MGS5 Fandom site where there's a hint in the manual related to completing an extra operation. Has nothing to do with the OG mantis fight solution.

a particular radio frequency could only be found on the box.

Contacting Meryl shortly before that fight requires getting her CODEC frequency, which is not told to you directly in-game. It's on the gameplay screenshots on the back of the box.

I heard the new James Bond game locks single player abilities and unlocks behind an always-online connection. You can play without it, but you can't level up or unlock any new skills or abilities.

Pirates might have to get more creative, but it's possible.

Proxies that scrape and save server-side data while you play, going through as many areas as possible, for instance.

Even if the communication between client and server is encrypted, the client can't decrypt if it doesn't have the key. And if the key is on your computer, it can be procured.

Reimplementing the server from scratch can certainly be necessary in a lot of cases as well, of course.