Larian's first duty is to make an enjoyable AAA video game, not a D&D 5E simulator.
If the 5E rules don't translate to a video game or are un-fun to play then Larian will change them just like any DM making house rules.
The main difference being, unlike most DMs, Larian's house rules are approved by Wizards of the Coast.
A useful quotation from the DMG:
The D&D rules help you and the other players have a good time, but the rules aren't in charge. You're the DM, and you are in charge of the game.
Larian is the DM and they are in charge of the game.
If the 5E rules make for a fun video game they will use them, otherwise they will change them as they see fit.