This heuristic has the advantage of having next to no false positives, even though it allows for lots of false negatives. Given that false positives are absolutely disastrous while false negatives are merely unfortunate, it's a trade-off I'm happy to make.
I guess the majority of repos are either bigger and useful open source projects or just small toys.
If you're looking at somebody with repos from the first category, most likely his resume will mention "author/ contributor to project X".
For the people in the second category, only thing you'd infer is that they can somewhat code. Which you could assume as well if they worked on any delivered project.
There are obviously exceptions in both cases.
Every heuristic overlooks good people.
This heuristic has the advantage of having next to no false positives, even though it allows for lots of false negatives. Given that false positives are absolutely disastrous while false negatives are merely unfortunate, it's a trade-off I'm happy to make.