Sure that can happen. Could be a long-running feature branch whose functionality is relatively isolated from other parts of the code. Could represent a lot of actual functionality.

But in most situations when I see a PR over a few thousand lines to an existing codebase that's highly depended upon, my stress levels tend to spike. On particularly complex projects and critical component, I'd read every single line.

Some modules I worked on, I would barely trust myself adding 100 lines to it because of how many other developers' code is depending on it. Sometimes they may be using it in ways which were not officially supported but I don't want to break backwards compatibility anyway.

So on such module, I would panic if I saw even a 1k line commit from somebody else.

15k lines is a lot. Each line comes with a probability that it introduces a bug. For a junior dev, the probability may be that 1% of lines introduce a bug... For a senior, it might be 0.1% per line and for the world's best coder, it might be 0.01%... But with 15k lines, and whatever the exact bug probabilities are, it's highly probable that the code contains at least one bug no matter who or what wrote the code.