That's really just not MSFTs game play is how I see it. They want an OS that can scale to various input and output methods. Something that is "futureproof".
So they need effectively, something with a small screen, that is in use. ATM they have windows phones. If that putters out, they will want something else.
In the long term, its hard to imagine people in 2050 using slab phones. They have been around enough to see computing go through many phases. I think their mentality is they want to be ahead of those changes, rather than chasing the present.
Does that mean they probably want to continue in mobile for now? IMO yes. In some form or other. Their continued work on win10m is suggestive that this OS forms some form of basis for ongoing work, whether as a core SKU, or as a part of something that will one day be merged.
I don't see them entirely giving up on small screens right now. But I also don't see them seeing that, as the be all and end all, forever future.
The interesting aspect of all this, is the merger, that is still a work in progress. Windows on x64, on arm64, and x86. UWP on HoloLens, on hub, and on phone. Its foundational work on an OS, that isn't dependant on the changing tides of technology. Something with its eyes on the horizon.
That's not a method that's always going to pay off in every field, nor is it something with immediate consumer gratification. But as a corporate strategy is one that hopefully avoids a sudden shift in technology from making their entire core business redundant, as could happen with too many eggs in one basket.
They might fail and re-entrench in some niche. Or they might succeed and win the future.
On the topic of silence - apple never pre-announced the iPhone. Apple has a secret AR project. Facebook a secret VR project. Microsoft is working on its Andromeda project, and google on fushia. Companies don't tend to leak too much information as to give their competitors a heads up. I think it likely that when it comes to phones, MSFT doesn't want to give to much away.
That doesn't mean their plans will be a wild success, or they have Skynet, sitting in a testlab. But silence is sometimes a nessesity.