There you are, talking on and on and on about some tiny unimportant but extremely specific implementation detail no one should ever have to care about. People shouldn’t have to read a manual about the inner workings of this captcha implementation (and have some experience with what types of text computer vision is good and bad at recognising!) to have any chance solving it.
In this case the author clearly had no idea how that control/unknown system works in detail (it seems like they, just like me, only know that you do not have to recognise both, but they didn’t really understand the reason for that – nor should they have to) but that doesn’t really matter for their argument even a tiny bit.
For me at least, the point would have come across better if that (seemingly) false ignorance were dropped. (Either that, or frame it in terms of, "Here's what an average user sees when they try to log in," or something along those lines.)
Except for some untrusted websites / users who can get really difficult captchas sometimes: https://i.imgur.com/6pAatnC.png
Also, '“Onightsl”? “Onighisl”? Are those even words?' No, my understanding is that dictionary words are never used as the control, so as not to be vulnerable to dictionary attacks.
Edit: I'm not suggesting that these captchas are in any way good; they do clearly have issues. I'm just saying that storyline in the blog post seems contrived. To me it would be more convincing if presented in a more genuine manner. However, perhaps he was simply very unlucky.