The effect doesn't seem that clear. That it varies from project to project makes me believe it's just some weak correlation. They should have tracked developer

The effect doesn't seem that clear. That it varies from project to project makes me believe it's just some weak correlation. They should have tracked developer retention to those projects. People might still contribute and fix issues if someone's being a jerk, but are they going to stick around and contribute further? That's the real question to me. Someone actively contributing for a project for years is much more desirable than someone doing drive-by PRs.