My point is: are you sure the candidates failed because of their programming ability (unable to use substring), or was they confused due to the question itself?

My point is: are you sure the candidates failed because of their programming ability (unable to use substring), or was they confused due to the question itself? It is very easy to get to the situation in a interview where the candidate is just straight up unsure on what the question is about. The candidate might fumble not because of not understanding #substring, but rather whether you would accept it as an answer. Many people would say those candidates should be failed anyway due to X, Y, Z reasons. But I want to highlight that in those cases, the interviewers should realize they are assessing something else, and not programming skill.