Personally I don't think you can measure difficulty in terms of "core rules" usefully 'cos video games and table top games work very differently; for example monsters are controlled by stupid AIs rather than evil DMs so they need to be buffed significantly to afford any sort of challenge, there is no perma-death so encounters need to be balanced so key encounters are more likely than not to result in party wipe on first attempt or the game will have no tension or sense of danger (you have to be scared your whole party will perish in a video game adaptation, not just your own character).
It would be extremely unwise of Larian to tone the difficulty so high that every single encounter is designed to be a party wipe. That's bad for several reasons.
1) A system overtuned to be a potential party wipe in every single fight allows for too room for things to go wrong. That's bad enough even if you have a high level of control over outcomes. It is "computer thrown out the window" frustrating when the control is in the hands of the dice and there's approximately a 50% chance to fail at every single thing you do.
1b) Some players will make mistakes because they're unfamiliar with the D&D mechanics. Some players will make mistakes even if they aren't. Enemy AI can make surprising moves. There can be surprises in encounters you are not prepared for. Even if your attack plan is solid, the dice can go against you. No room for error means no one will WANT to play the game.
2) If every single fight is a party wipe, then all fights will feel the same, no matter if it's against a handful of zombies, or a nest of mind flayers. The players won't feel any more tension for the final boss than they do for those zombies, which is death for emotional engagement. A player who is emotionally unengaged with the game will start to wonder "why am I still playing this", and then they'll stop.
3) It wouldn't be fun. Whether you like it or not, part of the reason people play these games is for the power fantasy (even if it's subconscious), and making the player constantly feel weak and helpless in every single fight is going to annoy them.
