Letting your intern blindly commit to your code base seems like the bigger issue here. The entire purpose of an internship is to learn and to be guided by professionals, not to be treated as a cheap laborer. You don't hire interns, you train interns.
Have you used copilot or are you speculating?
(And people couldn't tell in code reviews, nor on other occasions to see the code, since it's not normal to recognize GPL'd code on sight; everyone just assumed the developer was productive.)
This has not been my experience. I was dropped into the developer team and expected to know the entire tech stack and was not trained by anyone from the company at any point. Have I been bamboozled??