I mean, even if they don’t get hired they’re still contributing to open-source. Seems like a good outcome either way

Incorrect.

You are making sure your passions align with your work.

If you can find motivation beyond a paycheck and do it then you're more confident you'll enjoy it for the long haul.

Jobs are easy. Satisfaction is the hard thing.

Strictly speaking yes this is uncompensated labor but if that's all you see, let me help a bit.

What if the target is NASA, Wikimedia foundation, maybe some software for Doctors without borders? They all have salaried positions and volunteering is probably a solid way to know if it's a good fit

Why must passion be involved in work? Why can't it just be a profession?

Some people have passions that are not monetizable.

It doesn’t have to, but there is a good overlap of the vent diagram of engineers who open source their work with engineers who have a passion directly related to that work.

I can see both sides of the argument, some people want to just do their stuff and leave, whereas others might wholly devote themselves to the problem.