We've always known that Rainway will work on anything with a modern browser, Switch included. That's nothing new and it was always clear that a browser based solution would be able to happen if Nintendo rejects the app.
However, the browser solution isn't what's exciting (in the sense of "What? Nintendo is allowing this?") about this. What's exciting is the dev teams insistence that Switch will get a native Rainway app that will be downloadable on the eShop.
However, him now showing off the browser-based solution does make it seem unlikely that an app will indeed happen. I doubt Nintendo wants official developers to demonstrate how you can get around the browser's restrictions (not to mention why he would work with the browser to begin with). At best this might be a sign that Nintendo plans to accept web apps on the eShop, I guess, buuuut I don't think so.