Hmm, as andrewljohnson said, I expect that the posting impacts who applies, but I would expect that to disproportionately turn up applicants who can write C.
Another dynamic at play is the fact that those who don't have useful skills apply more often as they continue to be out of work, that doubtless accounts for some of it.
Some examples of coding questions I've asked in the past: reverse a string in place. Ones count of an integer. I'm testing if they understand pointers and bit manipulations for example. These should be answerable in 5 minutes max in 5 lines of C code. The incorrect answers candidates generate surprise me.
In my interviewing experience (looking at the time for something not dissimilar), lots of people think that 'knows C' and 'embedded systems' are the same as "I took a single module involving C at university, and by the end of it, my things were only randomly segfaulting maybe 70% of the time."
Or they've never touched either C or an embedded system in their life, but somehow it ends up on their CV and they tell you about how passionate for learning they are. Which may well be true, but it raises questions about your honesty and/or capacity for introspection. Also, bare-metal embedded really isn't the place to learn C[1], given that most failure modes are "It doesn't work. And maybe something caught just fire."
Another dynamic at play is the fact that those who don't have useful skills apply more often as they continue to be out of work, that doubtless accounts for some of it.
Still a bit strange.