I didn't know about condition variables either, but it seems like the questions are drawn from their own problem domain, concurrency (condition variables, multitasking modes, threads vs coroutines). It's fair enough to ask at an interview if that's what is used in your code, but I don't agree that 'what is a condition variable' is of the same level of 'fundamentalness' as 'reverse a linked list'. A graphics programmer will probably find people who don't know their transformation matrices by heart lacking in fundamental knowledge, too, but in the end it's just one concept that is taught once over a 4 year degree, and after that is only relevant for people in that specific subfield.
The article says 'Because they’re part of a core body of knowledge taught in any undergraduate curriculum worth its salt, and because in some form or another, they came up in our daily work' - I can agree with asking it because of the second part of that, but I don't agree that all the questions presented in the blog post are in the category of the first part.