It would be passable if he was playing a classic RPG with quests and character growth, but he's playing a modern gacha-based RPG-like nonsence at best.
And all his hidden knowledge comes not from doing some story-driven quests in the game he played previously, but because he's an obsessive teen edgelord, and I've got a feeling that game devs just tweaked the game sometimes when they saw him repeating same actions again and again obsessively, lest he offs himself IRL.
Like, what sane game dev would make those conditions for gaining new skills without much of an explanation?
I.E.: To get this unique perk I have to kill 3 dragons: 1st one - using only my 1st skill, 2nd one - using only my 2nd skill, and 3rd one - STRANGELY, using NOT my 3rd skill, but 4th and 5th skills combined. And it has to be done naked, on the night of 5th full moon of the year (only if it's raining). Something like that.