I have personally seen this happen:

Someone tried to contribute “vibe-coded” device support to a project I’m involved with, they said they did it all based on the device documentation, the code their agents spit out was copied verbatim out of a (GPL’d) project with which I’m familiar which supports that device.

LLMs are not learning things and then using that learning to construct new things. They are essentially a form of lossy compression of their training set. And you don’t need to be explicit about trying to reproduce a portion of that training set for an LLM to output one.