Under U.S. law, to prove that an AI output infringes a copyright, a plaintiff must show the copyrighted work was "actually copied", meaning that the AI generates output which is "substantially similar" to their work, and that the AI had access to their work.[4]

Wikipedia – AI and copyright

I've found a similar formulation in a official German document before posting my above comment. Essentially, it doesn't matter if you've ~~"stolen"~~ copied somebody else's code yourself and used it in your work or did so by using an AI.