As you mention, the proprietary software companies I have worked for are very different. By close of business on your final day you are locked out of everything.
I work for a company that fosters a community of our company alumni. Mostly by having a side chat alongside the main company chats, and arranging alumni meetings every now and then. So nothing that big really.
But it does help I think -- some people decide to come back later, there are perhaps new business opportunities for the company, you get to keep in touch with old colleagues. I think it's a very good thing to have.
Because open source is somehow inherent to a meaningful life?
As the article mentioned, work is important for life. It gives your mind varied experiences, which keeps it sharp. Open source means the value of your work doesn't evaporate once you leave the company.
Of course, you could try to fill 8 hours a day with something besides programming once your retire. But it's still strictly better to have the OPTION to continue programming, and even better to do it with some of the same code (assuming nontrivial codebases).
If you spend 30 years doing something 40 hours a week, you're going to be a lot better at it than something you only did on weekends. And at least for me, the reward is tied to the accomplishment.
For example, I've been learning how make videos with a DSLR recently. But this is at most a weekend thing. It's not that rewarding in the initial stages since I'm not good at it, and nobody cares about the end result because it's not good enough. With programming that's not the case.
If I were retired, I wouldn't be able to build meaningful work out of shooting video (or at least it would take another 5-10 years to do so). But I would be able to with programming, outside of a job. I could teach programming too, whereas I can't teach video.
But of course it's good to cultivate other hobbies... you could get crippling RSI and not be able to program. But my point is that work is privileged over hobbies. It's a different thing. Dawdling around with no goal is what makes people lose their mental toughness.
You don't want the entire value of your work and your social connections to be abruptly cut off once you leave the company. I saw this happen to both my parents. My dad was forced to retire at 53 and the value of his work was locked up inside that company (he worked there for over 20 years, so your skills develop in a bubble).
For the last year and a half, I've been working on open source sponsored by the company. (I've worked on open source since 2009, but this is the first time I've been paid for it.) Even though it's mostly "our" project, there are enough outsiders lurking / paying attention I know that if I were to leave the company, I might have some opportunities or people to reach out to, or even just people to talk to about technical topics.
Some of the proprietary software I've worked on has been very interesting, important, and technically sophisticated... but I know that once I leave the company, its value is drastically decreased. Basically you have to build up that value "from scratch" again, whereas open source can be forked, modified, combined, etc.