Evaluating CLJS in a browser requires a build with self-hosting capabilities. AFAIK, even these days it's still a finicky and not that well documented thing - not that many need that functionality, especially given that it inflates the bundle size. With that being said, it's no surprise that you can't get it working when you use a different template, a different build tool, and a different way to start REPL.
Also, that guide is more than 5 years old. Some things might've changed.
using the self-hosted stuff with shadow-cljs requires doing this: https://code.thheller.com/blog/shadow-cljs/2017/10/14/bootstrap-support.html
the stuff described in the other guide will not work with shadow-cljs