This would be (somewhat) true, except 95% of WP devices will be Nokia. However, the phones would have to be branded "Microsoft" for this to be correct. The phones will certainly not be branded Microsoft, they will be branded Nokia, maybe with a Windows logo on a button or something.

Objectively it would be more correct to say that MS will be an OEM for Nokia.

But the point is, none of the above will be correct, it is not very relevant in fact. The OS and the HW is together a vessel for the ecosystem. It may come with many different brands, hotmail, Ovi maps, Bing search, Ovi music, streaming music, downloadable music, different providers for Apps and music and so on. It is a complete ecosystem all by itself. It is a big mall for all kinds of things, and MS and Nokia rent out the space.