The only other cross-platform browser engine is Firefox, and I don't think it's architected to support a framework like NW.js, so it's not been done yet AFAIK.
Edge is Windows 10 only, so you may as well publish a Windows Store app (which you already can do). Safari is Mac-only, and I don't think it would be very interesting if you couldn't use it on any other platform.
For the sake of inclusion, you could do the same thing in the Mac app store by loading everything into a web view and publishing it as a native app. I believe this is the mac equivalent to a Windows Store app. If you have a game that runs well on MacBooks, you might stand a much better chance of getting noticed in the mac App Store than in the Windows Store because there is less competition.
Then again, NW.js works just fine for this purpose, and it is much simpler than creating your own project in Xcode.