Hard to believe that AMD would allow this, unless MS didn't have plans for an XB console that required a GPU from the Navi time period, or an AMD GPU at all. If Navi was developed with PS5 in mind, then it's also possible that maybe their "next gen" arch is being developed with XB Scarlet in mind.
If PS was able to pay for exclusivity to the Navi arch, I can only imagine they did this with one main goal in mind. To make sure they can force XB to end up another year behind the PS5 launch. There is a good chance MS would have known this for some time and that might be why they decided to wait and launch the stronger Scorpio prototype 1 year after Pro. That way both mid gen upgrades would have at least 3 years on the market before being replaced/overshadowed by next gen. This would also allow XB to use the most powerful console marketing with such confidence going forward, if they are going to continue to be a year behind every PS console launch.
If they wanted to launch alongside PS5, this could lead XB to pay a lot to have Vega customized, using more outdated GPU tech than the PS5 would have. This could mean Scarlet ends up weaker than PS5, or stronger but a much larger console physically, or slim but with another power brick. If XB went with the AMD "next gen" arch, it would be very hard and very very expensive to move up the launch to anywhere close to the PS5. It may also push XB to look to Nvidia for 2019 GPU tech to match the PS5 launch time. This could also be very expensive for XB and more of a headache.
This all leads me to believe that if this rumor has any truth, that you will end up seeing XB Scarlet showing up 1 year behind PS5. If XB is pushing to launch Scarlet by late 2020, the odds are good that PS5 is going to launch late 2019 if not early 2020.
PS1 - ! - We must build a console that can alert our enemies.
PS2 - @- We must build a console that offers online living room gaming.
PS3 - #- We must build a console that’s powerful, social, costs and does everything.
PS4 - $- We must build a console that’s affordable, charges for services, and pumps out exclusives.
PRO -%-We must build a console that's VR ready, checkerboard upscales, and sells but a fraction of the money printer.
PS5 - ^ -We must build a console that’s a generational cross product, with RT lighting, and price hiking.
PRO -&- We must build a console that Super Res upscales and continues the cost increases.
