The SerenityOS project literally has "NIH syndrome" at its core ["the SerenityOS project, which has a strong culture of writing everything from scratch.", https://ladybird.org/#faq] so I would expect a degree of skepticism of AI code generation.
But there was the post semi-recently about Cloudflare developers shipping a mostly-AI-generated software, overseen by real devs [https://news.ycombinator.com/item?id=44159166] Perhaps Andreas' use of Copilot is a similar thing, where he saves keystrokes and finds errors in the generated output with his own experience. I'll go watch some VODs.
They're still not using existing browser code because being an independent implementation of the web standards is a key goal. They've found bugs in the standard just by implementing it.