"Microsoft wants to surprise Sony this time"
That... this entire supposed timeframe is not how software engineering, OS engineering, or APU engineering works.
As a software engineer that has worked on 10+ APUs, I disagree - at least for my company.
Unit testing is essential for developing software and being that this is being mostly developed in house, I am sure that they have a wide array of unit testing environments set up. It's easy to design software for unreleased APU specs if you can simulate/emulate the environment - which we do as well as most tech companies.
We often design software for scalability before a product is released. Scalability for end users and well as hardware. If you are not designing your software with this in mind then you are setting yourself up for failure - this is a basic SWE principles IMO.
As far as OS's, they have been designing OSs based on scalability and functionality for many many years. I am sure they have this part down, same would go for Sony. I have worked very closely with MS and as far as preparing software for predefined or undefined specs, they are pretty damn good at that.