The term unicorn actually comes from the fact that they run the 'unicorn' webserver. When a webapplication error occurs unicorn catches it and displays an error to the visitor. They prettied that error up with a pretty image of a unicorn and a small explanation that something went wrong.
So when they say 'we saw unicorns' they literally saw (reports of people seeing) pictures of unicorns. Now I understand that might confuse someone who does not regularily use github, but it is just a term that is in their jargon and I see no reason why they should market-speak that up in an intermediate status message. (Note that these status messages are not press releases but reports that engineers make during the discovery process)
So when they say 'we saw unicorns' they literally saw (reports of people seeing) pictures of unicorns. Now I understand that might confuse someone who does not regularily use github, but it is just a term that is in their jargon and I see no reason why they should market-speak that up in an intermediate status message. (Note that these status messages are not press releases but reports that engineers make during the discovery process)