FOSS isn't communism, Foss hasn't eliminated class relations. Using an free open source library to make more money for your boss isn't communism. While I love FOSS, it's definitely not communism.

Marx never said people aren't nice and don't give things to each other under capitalism as far as I know, where are you taking that from?

And the existence of FOSS is reliant on a few key sectors which capitalism could very well destroy or mutate into something much different than what they are now. I don't see far-fetched the idea that the entire physical infrastructure of the internet will one day be privately owned, and companies will be able to decide who takes part and who doesn't, what kind of content is allowed... The fact that the capitalist overlords still haven't eliminated it, doesn't mean they possibly can't.