They're added manually at the moment (or the list is refreshed manually, I should say.
One of the first things I'll do is automate that so it is refreshed on a schedule.
The mentions plugin only works with the default composer. Redactor would greatly benefit from mentions working, as I'm sure many future composer plugins would too.
I assume it's got something to do with the classes it binds to, but can't be sure.
agreed, please @administrators make it happen