Summary: Microsoft's One Windows strategy is only causing confusion because we seem to have forgotten what an operating system is.
Microsoft CEO Satya Nadella laid out very clearly on the Microsoft earnings call what the strategy is for Windows, which is to "consolidate overlapping efforts. This means one OS that covers all screen sizes."
He even repeated it: "One, single converged operating system for screens of all sizes."
That's one OS, built by one OS team, for multiple different systems. What's 'one' about that?
Nadella talked of one team with a layered architecture which allows it "to scale the UI across all screen sizes. It allows us to create this notion of universal Windows apps and bring coherence there."