I just experimented with the usernames in that video. Most of theme exist (except for "v2v3").
But a lot have barely any repos. There is even a "ferran" who doesn't have anything (public) on GitHub.
So, my guess is that Copilot stored a lot of GitHub accounts, and when we type "@", it autocompletes with any random ones from that list.
There is no relation with the code that it generates.