I don't think that's exactly correct. There's several PS4 games that show extra stuff in options on PS5 that aren't showing when played on PS4.Xbox Ones games in BC can be aware of the Series X hardware, while PS games don't. MS BC solution is simply more robust and flexible.
Xbox has better BC in a sense, but I thought this whole thing was more because Sony's validation process for updates is a much bigger hassle than it is on Xbox. Like you'd need to release a new version of the game on PS5 to update it any more than what's possible when you crank the Pro version to max.
Like at least it has nothing do with Xbox BC performing better, as there are loads of examples where BC games perform similarly between SeriesX/PS5.