Not many people who are trying to screw their party over will openly admit it. Boosti did give off the impression that he was not going to stop/didn't stop playing ballad though. Not saying he was trying actively to screw the party over, but he was seemingly refusing to listen to their reasonable request judging by what transpired in his own words.
You can't really say RiceisNice is being toxic just because they're bringing up some inconsistencies in the story, and RiceisNice has already said they may not be understanding the whole thing. Whether that's because of the rest of the party's side being left out, or just bad wording on Boosti's part, who knows. But the disclaimer has been made.
Boosti on the other hand has made some direct personal insults towards RiceisNice when they have already said they may not be understanding the story right, just because they gave gave criticism about Boosti's choice of playing ballad. Criticism that was correct and delivered civilly.
And if Boosti did continue to play ballad despite the party clearly not wanting him to (it's rational to think he did, since why else would the party get mad at him later on again when he hadn't said anything?), despite it affecting the party negatively, then it isn't toxic on Rice's part to call him out for continuing to do it and hurt the party. Refusing to listen to the party's wishes or listen to advice is toxic behavior. They could have been nicer about it, sure, especially if their first reaction was what Boosti claimed, but they were right that he was sacrificing considerable dps by playing ballad when it wasn't needed, and therefore hurting the party. Refusing to stop doing that is toxic, and in a way, intentional. Ignorance of job function may not be, but ignoring the party's reasonable request is intentional.
Also don't see why you think Rice thinks they're "god's gift". They gave valid criticism of the choice to play ballad, asked about/criticized inconsistencies, mentioned the 2nd party's side of the story, and even admitted they may be wrong.