How did it go from "He even performed a backup/mirror of several dozen of our repositories." to "He deleted part of our repository from GitHub. (..) [He published] an empty package in the cooker repository, which obsoleted all gnome and cosmic packages."?

I feel like there's a few steps missing there. How does it go from "a new person joins the community" to "he's able to nuke everything"? Sure, he might be reasonably well-known, but in the post it doesn't sound like he was a core maintainer, or even a very active community member. Do they just randomly hand out admin access to anyone?