It's the same with XBox BC, except you don't buy the game. You still need to download a version compiled for the One and Series. The disk only prompts a download. So by your definition XBox One and Series don't have BC at all, just free updates for lazy remasters ;p
When you typed the "except you don't buy the game" should have been a dead giveaway for you. Lol.
...to avoid getting banned for inactivity, I may have to resort to comments that are of a lower overall quality and or beneath my moral standards.