I think one of the reasons it's a useful indicator is because most (if not all) startups require a huge level of commitment and focus from the founding team. If you aren't able (for whatever reason) to commit to it, it's a sign that you might struggle to be successful.

I'm not arguing that people who don't quit their jobs don't have amazing ideas; it's just that to make a startup work as a business requires so much effort, it's likely that if you're not able to quit to work on it, you'll lose out to a team that can.

PS: I'm saying this as a founder who quit his job (as did my co-founder) when it looked liked we had something, and I doubt Overleaf would be where it is today if we'd not. So I may be biased in that direction!