I don't think it's unreasonable to think Swen is being nice and political for professional reasons here. It certainly doesn't rise to the level of conspiracy theory.
It's very unlikely. If Swen was actually ditching it due to Wizards of the Coast not being nice to him that he wouldn't have said anything. Historically when there's animosity between companies they don't talk about it at all or cite "creative differences" as the reason why. They almost never say "no actually we're good" publicly. Swen also says a lot of things that would be weird if he was just making the whole thing up. Things like how when he cancelled DLC plans his developers celebrated because they were more excited about the next project he wanted to do instead. That sort of thing would be very easy to debunk if an anonymous whistleblower came out later.
Additionally historically it's rare this happens in video game design. Like Bioware for example abandoned Wizards of the Coast and Lucasarts primarily because they wanted to make new IPs instead of hitching a ride on another company's ip. Which led to Mass Effect and Dragon Age.
I think it's much more likely that Swen was telling the truth and that Larian just didn't want to keep making D&D based rpgs. 5 years of BG3 was enough for them. And that they actually are working on something else. I think speculating on that is more interesting. He said for example it's not DOS3. I also personally don't think it's Divinity as that likely wouldn't make his team celebrate.
In my opinion there are three possibilities
1. They're working on an entirely new IP. Likely not a medieval fantasy game
2. They're working on another licensed sequel to a beloved rpg. Something like Arcanum or Vampire the Masquerade
3. Swen somehow managed to convince EA to sell him the Ultima IP and got Richard Garriott to license him Lord British.
