I will read the thread and reply more comprehensively later.
But let's assume that Kepler is right and AMD CEO and Xbox President both are liars, that there is no BC with Xbox branded devices on the same family of APUs as Magnus.
That doesn't mean there can't be FC (Forward Compatibility) whereby all new titles published on Xbox, in order to gain certification for publication have license to run on all Xbox branded products capable of doing so.
Nothing prevents MS from updating the licensing agreements for new games released after next gen hardware to support the entirety of Universal Xbox Platform.
Where the same GDKX created and optimized for Magnus game can run on Xbox devices.
Up to 4k/120 on Xbox PCs, Xbox Consoles, Xbox Cloud using Magnus, and up to 1080/120 on Xbox Laptops, and Xbox Handhelds (and also tablets or mini PCs).
There is nothing preventing MS from updating and securing Series console BC via each publisher, the same way they do for streaming rights.
For example, MS just secured permanent streaming rights for GTAV to be streamable via Core and standard tiers and xCloud Buy and Stream. MS can incentivize publishers, MS can use the carrot and stick approach.
Start with full BC, let publishers that whine, Opt out. Then buy those publishers out, either literally or just some powers of persuasion.
The complete Xbox platform with Seamless NATIVE gameplay will be created one way or another. It's a hardware based platform based on AMD APUs. Play Anywhere will be used to cover ALL other types of PC devices.
MS will have FC and BC can be done on case by case basis.
That's the entire point of the AMD long term partnership.