I plan to write an article about the tool, as if some people already did.
StimulusJS is to be used with advanced ES6 class features.
This is why it doesn’t work yet without a proper build system, the documentation needs to be updated on that point.
You need either Webpack, or Babel with a special configuration, or Coffeescript (which also transpile into ES3).
Just insert Classtrophobic amongst the scripts, and the code example of StimulusJS works with very little adaptation: