I'm not sure if I follow. Reading your post it seems you managed to write a plugin but looking at the repo you linked the plugin lacks the entry point and the logic you want to include.
It is just an "ical" module you picked somewhere.
If you want to write a plugin I would suggest you to look at plugin examples:
https://github.com/koreader/koreader...master/plugins
The "developer docs" you mention are not there as an alternative to read & understand chunks of lua code.
There's no API for plugins. Plugins can do whatever you want, there's no isolation between plugins and "core". Everything shares a state.
I would suggest to start reading "hello.koplugin" and shift focus to other non-trivial plugins as you manage to understand and implement the chunks of code there that are required for your plugin to "show on the UI" and "do something"