The first example shows the server rendering a handlebars template and then sending that as a response to the client -- it's then stated that this "isn't true SSR"

Then the same thing is done without a template language, using strings instead, and this is some different kind of SSR altogether and the "true SSR".

Which also seems to insinuate that only JS/TS are capable of SSR?

  Server-side rendering! Well, kinda. While it is rendered on the server, this is non-interactive.

  This client.js file is available to both the server and the client — it is the isomorphic JavaScript we need for true SSR. We’re using the render function within the server to render the HTML initially, but then we're also using render within the client to render updates.