Lol. I can't think of a single person out here who doesn't want the game to be fun and for Larian to make more fun games.
You tell me I'm wrong, but what do you base your RIGHTness on? I base my statements and suggestions on 25 years of RPG experience + a gazillion hours of playing videogames including almost every D&D game that's existed since Commodore 64 Bard's Tale and Pool of Radiance and all the old cRPG word games.
Since the freaking dawn of D&D cRPGs, I can't think of any that has so flippantly disregarded the source material so much, turning intellect devourers into thug grunts and imps having no resistance to anything and so on and so forth. Now, that said, it's EA, so maybe they plan on fixing that later. Who knows? And, in spite of it, I still love this game.
Yes, there is some room to argue all day long that many classic D&D cRPGs had only 4 party members, but that doesn't matter here. What matters here is what makes the most sense for this game.
I've given multiple reasons why a 6 party member BG3 is a big plus and why 4 severely limits. I've also explained how they can make it work without an issue. All you have done is get knit picky with me about the PHB and what it says about building encounters. The "rules" in the books for such things are guides. In practical use, I'm telling you, you can build encounters for 4 or 6 and it works out the same in the long run. Yes, in tabletop, you CAN design encounters based on player level and make sure to award experience so that by the end of a campaign a party of 4 is the same level as a party of 6 might be, but you can also design it the way I suggested.
And, just so you know, at one point, I even suggested that they could implement logic that does analyze your overall character level and tweak encounters to match. 3 imps against character level of 3 and the game should reduce imp health to 2 HP a piece because they have resistance and would require 4 HP of damage each to kill. It wouldn't take much to implement that logic, and then it would fit right into the PHB guidelines you quoted. I would actually prefer it. As a DM, that's how I would do it.
But that, again, should have no bearing on party size. Why? Because a computer can be programmed to modify encounters to match PC levels. You can even have demons fighting level 1s with legit rules IF the DM sets up the encounter right.