Thanks! 
The main reason is I’m far more familiar with JavaScript and Discord.js than I am with Ruby and writing Discourse plugins.
HOWEVER, it isn’t super difficult to run the bot on the same server as the Discourse instance and run the two behind an nginx proxy (in fact, that has a benefit since nginx will handle your SSL - the bot’s integrated http server doesn’t support SSL).
When the bot is ready to go out of testing, I’m going to write detailed wiki pages on the GitHub repo on how to configure the bot to run alongside Discourse on the same server.
Another reason it’s not a Discourse plugin is, if it weren’t for the integrated web server, the bot would be completely independent of Discourse other than API requests - the web server is only there because… your SSO callback payloads need to go somewhere!
Also, @erlend_sh gave me a feature request in PMs - support for custom groups. I have successfully added that. I used the built-in Staff group for testing.
I noticed that the Discourse API doesn’t treat the Staff group the same way as admins/moderators/trust levels. While all those aforementioned groups do end up in the user.groups list, those groups are also represented in their own fields in the user object. trust_level being an integer for your trust level, admin being a boolean, and moderator being a boolean. The staff group isn’t treated in that way - so it’s perfect for testing custom groups.