K_sze's point was that it is a lot harder to prove you have not seen and were not inspired by publicly available code than it is to prove you were not inspired

K_sze's point was that it is a lot harder to prove you have not seen and were not inspired by publicly available code than it is to prove you were not inspired by code that was kept secret. And while someone using free software in a non-free fork can release the changes as shared-source leading to the same problem, this has rarely happened.