Fair enough. I didn't mean to suggest that these captchas are in any way good. Only that the author does appear to have technical knowledge of how they work - otherwise they wouldn't use terms like "control word", so it seems that the difficulty experienced was likely contrived for the purpose of writing the blog post.
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.)
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.