By that narrow and exclusionary definition of “immersive sim”, almost no games regarded as “immersive sims” are immersive sims!
I mean you might, just possibly, squeeze Ultima Underworld 1 (not 2), S.T.A.L.K.E.R. under that bar. You would not be able to squeeze UU2, either System Shock game (but again, especially not System Shock 2), any Deus Ex (very much including the original), any Thief game, either Dishonoured, any Bioshock game (I actually think they are borderline anyway so no great loss but…), or Prey under it though.
Subnautica is an immersive sim in exactly the way the vast majority of immersive sims are, in that there is an essentially linear plot and events which will happen at certain points (in some immersive sims they can change, not so in others), but the player is driving the exploration and most of the stories you tell about the game are the results of player actions, often emergent ones.
King’s Bounty was a great deal more competent at being funny/silly than D:OS1, so it seems unfair to compare it. It knew where it was coming from and the writing wasn’t stellar but it was good enough to make it work. It also wasn’t a focus of the game.
I don’t think it’s really a cultural issue at all, either, especially given Belgians are about as similar to Russians as British people are, culturally. It’s a bad writing thing. You see it in the writing of primary English-speakers all the time. Most people just aren’t very good at writing stuff that’s funny. They fall back of lazy stereotypes or ridiculous situations that might raise a slight, almost reflexive smirk, but don’t actually make you laugh. Just read some fan-fic of something that’s funny and you’ll see the “jokes” are typically exactly the same quality and style as D:OS1 - terrible.
There’s a reason my primary point of comparison for D:OS1’s writing was “Bad Terry Pratchett fan-fic written by a teenager”, because that’s exactly what it reads like. They know what they’re trying to achieve, but they’re unable to consistently or even frequently achieve it, so just drop to “silly shit”.
Another place you can clearly see this, and one even more apposite, is in adventures written for D&D. I’ve been reading them since the 1980s, and whilst most are serious-ish, there are ones that try to be funny. Of these, perhaps 20% mostly succeed. The rest end up just like D:OS1, using cheap stock characters and “SO SILLY!!!” or “LOL SO RANDUM!!!” situations as if they were actually funny.