I'm fairly sure its explanation of integration is correct? Maybe not a good answer for a math major, but it's a good answer for his question.
GPT is confusing the fundamental theorem of calculus with the meaning of integration.
ETA: what makes LLMs so worrying is that I know calculus quite well, and I think GPT-4 is dumber than a mouse, yet I still felt gaslit by GPT-3.5's wrong answer! I had to make a mental picture to make sure I wasn't contradicting the Mighty Talking Robot. This technology is poison.