As someone who works in game development, I can tell you that this makes absolutely no sense.
Game developers do not care how you play the game solo in single-player sessions, because you’re only changing the experience of the game for yourself.
It only bothers game developers when people try to cheat online and ruin the experience for their other paying customers.
Subnautica are not deliberately trying to kill off trainers from working in their game.
They even have built-in cheats: Subnautica - Console Command Cheats. So they’re clearly not anti-cheat developers. 
The trainer is just quite old now and most of the playerbase has moved onto other games. Because, let’s be honest, this is an amazing game but once you’ve played Subnautica once or twice there isn’t really any more great replay value, is there? Because everything has already been seen and done. Unfortunately.