This is not just some art thing. People note it in every field. It's not the only predictor; the author of Chrome reportedly did most of it offline IIRC. But it

This is not just some art thing. People note it in every field. It's not the only predictor; the author of Chrome reportedly did most of it offline IIRC. But it's a real phenomenon that is robust in the face of the concerns you're raising. There is a wide range of email responsiveness -- even among people who are going to the bathroom and putting on birthday parties and doing focus work -- and it is a helpful predictor.