Sounds like they might open it. But probably it won't be as useful as Wine which is portable to POSIX systems. May be it can benefit ReactOS or something.
What could be good though is MS backing the Wine project. But I guess it goes too much against their lock-in DNA.
This is a "Windows Subsystem" implementation. There are no open subsystem implementations and it hasn't been documented. Therefore it's a little far-fetched that they would release this as open source. Not impossible though.
And here you've hit upon what would actually be interesting: the open sourcing of Windows itself, open sourcing of Hyper-V, etc. This is a 'nice to have' addition to Windows for sure. But it really does nothing to make Windows more relevant as an open source platform which is actually a major technical strength of most UNIX-alikes. And no I'm not saying Microsoft has to go GPL.