This is definitely true. I'll often check to see if a candidate has a github account when making a hiring decision. Having some interesting/good code up there can definitely move the needle a bit on a hire/no hire decision.

Absolutely. A record of solid past work can help overshadow a poor interview (nervousness, etc.) in my eyes.

If I can see the sorts of things a candidate has worked on in the past, I can more tailor the interview and ask more engaging questions. I want to know what they're passionate enough about that they will give up free time to pursue it.

None of this is specific to Github.