The XBOX ONE S *is* a Series S console.
The XBOX series 'S' is a series S console.
Hence the backward compatibility is from the previous series S console. It's not rocket science. Same series same games.
There are a lot of sites running this none story but your headline is amongst the worst.
This headline is at best incorrect and at worst a flagrant attempt to get visitors using a distortion of truth.
Please stop posting this sort of thing it diminishes your brand.