You've kind of jokingly confirmed your own suspicion. Dev confirmed that the MS GDK includes scalability from the XSX down. Very little work on the Dev side as MS gives the tools to begin work on the XSX then scale down to lower specs. Should be even easier now that the XSS is rumored to have the EXACT same CPU as the XSX. A bit less ram and CU's leaving me to suspect that it's an all digital "gamepass box" if you will
Not everything scales in a linear way.