I don't believe this falls into Eureka moment category. This is adding an operation into a codebase which already implements a similar operation. The problem i

I don't believe this falls into Eureka moment category. This is adding an operation into a codebase which already implements a similar operation. The problem is also not "fluency in C" because they require it for day to day work. If anything sucks about this task it's that it also tests "set up a C IDE from scratch on Linux". This might reject a good candidate who nevertheless didn't recently set up such an environment. OTOH this one is easy to fix - either set something yourself or tell the candidate they will be expected to do this, so they will have time to prepare.