But how do they know it's vibe-coded? It may have a smell to it. But the author might not know it for a fact. The fact it's vibe-coded is actually irrelevant the size of the request is the main issue.
And if they did in fact spend 6 months painstakingly building it, it wouldn't hurt to break it down into multiple PRs. There is just so much room for error reviewing such a giant PR.