I doubt Deadfire "dumbing down" was the reason for its failure to appeal to casuals. Again, that's Codexers projecting their own grognardish discontents onto how the universe works.
DOS2 was indeed a up-front celebration of what the vast majority of casual purchasers liked about DOS1, though, and that is important. In fact, DOS2 I would argue was dumbed down from DOS1: it failed to build any more of a robust, replayable system, and it fails to recapture the Ultima inspiration that made Cyseal the best part of DOS1. But it didn't matter, because there are millions of casuals who loved DOS1's explosive environmental effects and a big world with a sense of adventure set against an uncomplicated fantasy world full of cartoony art and dorky writing.
Deadfire failed to do this every way, because the reception from POE1 was far more mixed and contradictory, whereas Larian got a very clear message on what the new casual funs loved about their first game. And then Deadfire did try to put new things in there to entice players, but it seems like the pirate schtick, etc. wasn't marketed very well (even leaving aside what it does to the actual narrative cohesion of the game).