I don't know about the linux kernel source, but having contributed to several major OSS libraries including Pytorch, I think that most PRs in my experience can be more easily described in natural language than in code. When I said comments, I didn't mean like line by line comments of everything, but I was more thinking of docstrings. I am very sceptical of the idea that on average it is faster to write complex code than to describe what you want it to do, which is partly why I think AI code synthesis can achieve significant speedups here.