Except, that's not what happens. In basically every coding interview in my life, it's been a gauntlet: code this leetcode medium/hard problem while singing and tapdancing backwards. Screw up in any way -- or worse (and also commonly) miss the obscure trick that brings the solution to the next level of algorithmic complexity -- and your interview day is over. And it's only gotten worse over time, in that nowadays, interviewers start with the leetcode medium as the "warmup exercise". That's nuts.
It's not a one off. The people doing these interviews either don't know what they're supposed to be looking for, or they're at a big tech company and their mandate is to be a severe winnowing function.
> It isn't that your interviewer knew all the languages, but that the language didn't matter.
I've done enough programming interviews to know that using even a marginally exotic language (like, say, Ruby) will drastically reduce your success rate. You either use a language that your interviewer knows well, or you're adding a level of friction that will hurt you. Interviewers love to say that language doesn't matter, but in practice, if they can't know that you're not making up the syntax, then it dials up the skepticism level.