Is this aspirational or anecdotal, or is this what technical people in FANNG/tech actually do? I hope it's true but it strikes me as the kind of thing that most technical people involved in the interview process would be too tired/overworked to do.

I agree. As a technical person who has been involved in hiring, I never looked at github. My evaluation of a candidate was based on how he/she answered questions in the interview, and my general sense of "could I work with this person every day." I had no spare time to go beyond that.

Nobody ever has brought up my GitHub though I did use my private GitLab projects to land my first dev job.