That said, I don't really find the critique that models have browser source code in their training data particularly interesting.
If they spat out a full, working implementation in response to a single prompt then sure, I'd be suspicious they were just regurgitating their training data.
But if you watch the transcripts for these kinds of projects you'll see them make thousands of independent changes, reacting to test failures and iterating towards an implementation that matches the overall goals of the project.
The fact that Firefox and Chrome and WebKit are likely buried in the training data somewhere might help them a bit, but it still looks to me more like an independent implementation that's influenced by those and many other sources.