If a username is completely inactive, GitHub support is pretty good about releasing the name if you ask.
first thing I noticed too, hah.
my small github claim to fame is writing its article's first revision on en.wikipedia, which may have been sent to the deletion kangaroo court for notability.
It looks like they've been around since 2010, which surprises me because GitHub was around for a year or two before that iirc. I still envy that username, though!
Sometimes people assume the low character count usernames are taken and don't even try. I've snagged some cool twitter accounts this way (not low though but like regular words).
I can confirm that. I grabbed "odb" back in the day from a squatter (no repos or activity)