We’ve already had NT + Linux userland; that was WSLv1.
I have no idea what they're planning or why, just guessing, as they seem to be bringing Linux and Windows closer together all the time.
This requires NT API compatibility due to applications using NT API. Despite Microsoft telling devs don't use the NT API, devs use the NT API and Microsoft makes adjustments to ensure compatibility.
> I have no idea what they're planning or why
Clearly, because the whole idea not only makes no engineering sense, it makes no financial sense. They need to build the NT kernel anyway -- it runs the entirety of Azure services!