The point isn't that agent output is magically better; it's that reviewing your own agent's output is way cheaper (intellectually) than reviewing a stranger's, because you've written the plan by yourself. And 'slop' is mostly what you get when you don't have a clear plan to verify against. Maintainers writing detailed specs for their own agents is a very different thing from someone vibe coding a feature request
You’re assuming that maintainers have a desire to use agentic coding in the first place.
Secondly, it would seem that such contributions would contribute little value, if the maintainers have to write up the detailed plans by themselves, basically have to do all the work to implement the change by themselves.