When you use isomorphic rendering, exactly the same code runs on the server, that runs in the browser. Usually you do this to render the first page load on the server, and then the client takes over from there, but for people who don’t have JS, they continue using server renders.
You said the following:
By using isomorphic rendering, you’re giving yourself a fallback, without doubling your work, because the same code runs on the server and the client.