All they did was graft whatever 5e rules would fit on the DOS2 engine, adapted a few rules to jury-rig DOS2 features in to the game, and then threw the rest out in favour of DOS2 mechanics.
BG3 is just a heavily modded DOS2.
BG3 feels like neither BG nor D&D, but it sure feels a hell of a lot like DOS2.
I agree that, so far, it doesn't feel a whole lot like the previous Baldur's Gate games. But it definitely feels like D&D. It feels more like D&D to me than most of the D&D games ever made.
If this wasn't called Baldur's Gate 3, and wasn't officially called a D&D game, and people played it, they would be saying, "This is a D&D game!" In fact, Larian would be getting sued by Wizards of the Coast for how much of the D&D brand suffuses every bit of this game.