ESA - mock with Mirage when using implicit grant authentication

Did you mean /config/environment.js ?

Yup sorry, typo. Would look something like this:

if (environment === 'mirage') {
  // enable mirage
  ENV['ember-cli-mirage'].enabled = true;
}

Just need to enable mirage. Could also turn on mirage logging (ENV['ember-cli-mirage'].logging = true) or whatever else you wanted.

Where did you define this mixin ?

See “Create authenticated route mixin” in my answer here

Or are they just to be replaced by some fake values ?

Exactly, they can be anything since it’s all fake and mirage won’t be checking your token