Yes, an agent wont tell you if something is boring or funny, at least not unless you ask (and i don't think its response would have much value) but it will tell you if your instructions can be followed or not - which was the main thing the author wanted to test against, at least as far as i understood.
My point wasn't that you can use agents for all feedback, but for the given case of testing your readme file's instructions (the article's title even mentions it is about the readme file) you can certainly use agents for that.