Your argument is that because FOSS would be permissible in AnCap society, FOSS being fundamentally constructed upon AnCom principles of rejecting Capitalism and centralization in favor of decentralized and collectively owned and distributed property makes it not in line with Anarcho-Communism.
When the article is giving an example of how Anarcho-Communism would work, Linux is a fantastic example. Nobody is saying Linux is Anarcho-Communism, or that Linux cannot exist within broader contexts, but that in an Anarcho-Communist society, the structure of Linux and FOSS would be the common structure.