If they would have used react-native-web, he would have saved some time writing the Android (or IOS) apps. Because he could have reused the code (he said that too).
This is what I have done, anyway. Also react-native-web is very very high quality library. I am just amazed at the quality Nicolas Gallagher is putting out.
My app now runs on web and via browser on android/ios with all the magic of responsive UIs, reshaping all those positions when a user rotates the screen or zooms out.
I am wondering, on occasion why I spent time on native apps (even with React Native) -- at all, as React Native Web (with a bit of React-bootstrap) -- are just amazing.
Wrt next.js, at least when I looked at it mid last year, did not support routing with React-router. And for me it was a big deal, I did not want spend time on learning specifics next-js routing.
Instead I went with https://github.com/jaredpalmer/razzle which supports React Router for routing, which I use Android/react-native and React-native-web, uniformly.
React-router, is also very good and useful library with nice, intuitive architecture for routing.
At the end, I decided for my need the Server Side rendering is a later-stage item. So I have not spend more time on it.
I suspect Spectrum did not loose much traffic because their app is not using SSA routes.
Zulip chat app (may be they are spectrum's competitor, not sure) - uses React Native for mobile (https://github.com/zulip/zulip-mobile ), but something else for web. As a reference.