>Can't have been much, as the Chrome project is 20+ years old and LLMs have only recently become a thing

That's not a great argument. GPT3.5 was launched over 4 years ago, so that's already significant compared to Chrome's age. LLMs notoriously also produce more verbose code than a human, so its not out of question that it might produce 5x more bugs on average given the same time frame.