Looks like MS are going for compatibility as a real selling point with the Series X. A good move considering how many games people will have and they say there will be improved performance with the older games too. Not sure about "the most powerful" line though.
news.xbox.com
Xbox Series X: The Most Powerful and Compatible Next-Gen Console with Thousands of Games at Launch - Xbox Wire
[Update 9/14/20: We’re happy to share that backward compatible titles will be coming to Xbox Series S as well as Xbox Series X. To deliver the highest quality backwards compatible experience consistent with the developer’s original intent, the Xbox Series S runs the Xbox One S version of...
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