[afloatboat]

Well, in Github's case the icons always seem to accompany a label, which would already make the content accessible. SVG also provides descriptions and titles that can be read out by screenreaders. Icon fonts are 'hacked' in specific unicode characters and often screenreaders try to read these characters, leading to nonsense.

But to specifically enter the question: They hide the icons. The aria-hidden attribute makes sure that the icon is not seen by screenreaders [1].

[1] http://www.456bereastreet.com/archive/201205/hiding_visible_...