Being an open-source developer was his biggest career mistake

I find it strange that individual persons still think of getting into open source as a profession. If the motive is to share something you built with others with no expectations (except attribution, maybe some bug fixes, a user community) in return, then by all means you should get into open source. But what rational person would think that working on open source full-time is a sustainable way to make a living.

Imagine a doctor who only does pro-bono consultation. Would they be able to make a living?

Agreed. Most interviewers (even technical peers) have never bothered look at my open source projects and contributions. What really worked, sadly, is the usual sauce keywords you serve (previous known employers/clients, technologies "you know" and how many years of experience you have), "the technical test" you give them back, and "closed source products" that you've worked on and are still online.