It isn’t using Web Intents, that is an [experimental, deprecated, Chrome-only] API to allow Android apps to open directly from a web page link. In this context, intent is what the commands you define for this specific Firefox extension are called, they have nothing to do with Web Intents – you specify what your intent is when a specific voice command is made
Basically, what is explained on that README is the extent of the documentation on them: that API exists inside that project, to allow you to specify things specific to that project