I'm gonna both agree and disagree with you. Certainly the open source phenomenon could never originate from a "Marxist-style" socialist economy, but it is identically the same as the "community" economy practiced in frontier days (think of barn-raisings). One gives of his efforts to others in hopes of gaining a similar contribution FROM others at some future time. I think you would be hard put to say that THAT "economic style" is "post-capitalist".
I would maintain though that in the old days folks did this because they didn't HAVE the "money" to do this sort of thing and needed help in raising a barn, for example.
I hold that what motivates folks to contribute to open-source is a lot more complex, since in many cases folks contribute more as a hobby than expecting a tangible reward such as quid-pro-quo, although no doubt the idea of "giving something back" is very strong if one has gotten good use of some piece of open-source software in the past...
It should also be noted that even beyond the issue of generating 'goodwill', there's another reason for people to share their work. Suppose I think GIMP is a wonderful program, but it would be more useful to me if it could do X. If nobody else has modified GIMP to do X, making the improvement myself would afford me an immediate benefit: I'd now have a version of GIMP that could do X like I want.
At this point, I can either share the software or not. If I don't share the software, then any time someone else makes any improvements to GIMP that I'd like to use, I'll have to merge those improvements with my own. If, however, I instead release my improvements to GIMP, then I stand to lose very little but can gain at least four big ways if my improvement is a good one: (1) If my improvement is popular, it will likely get incorporated into a mainstream version of GIMP so I won't have to patch it into newer releases anymore; (2) If there are any bugs in my improvement, others may find them and tell me about them (and maybe offer a fix) before I would have found and fixed them myself; (3) Other people amy be able to further improve my improvement; if they do, I'll get the benefit of their having done so; (4) If my improvement to GIMP is a good one, publishing it reduces the likelihood of other people making changes to GIMP that would be incompatible with my improvement.
Thus, even someone who is merely trying to get work done, and for whom issues of goodwill, self-promotion, and pride are all irrelevant, can stand to benefit from involvement with open source software.