Parity here isn't on a scale. It's a binary trait. Either they are the same or one is worse than the other. The shitty XBOX does not have CPU parity with the real one, and it's a serious limitation that effectively means that the "good" Xbox also has that worse CPU in terms of game design. It will obviously still get some games, but it's losing games that it would otherwise get because it has nothing in common with a next gen system.
Split screen being the specific thing that BG3 is struggling to do isn't the point. It's merely a symptom. For a next gen open world game, split screen BG3 is still not that demanding. The fact that all the real action is turn based makes it far easier to make run than a similarly dense real time game with real time physics demands, and the fact that the Xbox S can't handle it is a very strong example that it's a piece of shit.