But you can download Chrome, Firefox, and Opera on iOS
Yes, but they are not the rendering engines of those browsers (chromium), they are the ios rendering engine (whatever its called), like monument browser and UC browser in the windows store are the edge rendering engine.
There is zero stopping google or anyone else from doing what they have done already on ios, and many have already in the UWP, and make a rendering engine, secure, compliant browser.
The point is the original post is 100% wrong, there is nothing stopping anyone from creating a browser in the windows store.
Having a standard for rendering websites is a good thing, it adds to security and it helps enforce html5 standards (which chromium engine does not, which is why some browsers don't render some pages correctly)