Can i redeem a xbox 360 game that is backwards compatible on xbox one/ series s & x?

That's because it's a 360 game that's backwards compatible with the Xbox One and Series S/X.

The 360 version of that game is not backwards compatible. The one listed on the list is for the Xbox One and is compatible with the Series S/X.

The $29 one you saw is the Xbox One version of the game which is backwards compatible with the Series S/X. The 360 version is not backwards compatible.

I totally understand, however, the game has its own version that can be played on Xbox One or Xbox Series X|S and the 360 version game is not backward-compatible, even if we want to it is not designed to be played on the Xbox Series X|S console.

Yes, if you purchase the game from the Xbox 360 marketplace, you can play it on the Xbox 360, Xbox One or Xbox Series X|S.

As for what Hardcore Games said, you can still buy Xbox games for the Xbox 360, and play them on the Xbox 360, and if they are backward compatible, play them on the Xbox One & Xbox Series X|S.