I do not think "amateurs" is a good description about the people writing the code - most will be highly technical people with lots of experience. And "loosely coordinated" can be applied to many "corporations" as well.
I think it matters that people coding in open source do it because they care (similar to your idea but on the positive side). If you want to make something nice/working/smart you have more chances to succeed if you care than if you are just being paid to do it (or afraid that you will be embarrassed)
in this case would a programmer with a day job that also hacks on linux in their free time be a professional at work and an amateur on anything they do independently? Or really any sort of engineer, contractor, person who makes stuff, etc?
Just my interpretation: that programmer is a professional. They are paid to do programing. They are still professional even when they are working on their hobby project, because it is not a function if they are paid for that particular code, but if they are paid for any coding at all.
If that programer would go and coach their friend’s basketball team for free they would be an amateur coach, but they are still a professional programmer even while coaching.
I think it matters that people coding in open source do it because they care (similar to your idea but on the positive side). If you want to make something nice/working/smart you have more chances to succeed if you care than if you are just being paid to do it (or afraid that you will be embarrassed)