Actually it is quite small dataset by current standards (millions of images), so I guess it was using humans in the loop.
the relevant part is the masks, and so far it's the biggest dataset in existence, with about 400x more masks than the next smaller one.
: excluding non-public ones
they described it in their paper: they created it themselves. they had/have a loop of having humans annotate images their system was uncertain about, but the released one was fully automated.