There are several points to be made here.
First, good on the examiner for recognizing that was on them, not the student. I detest when an instructor makes a mistake and then tells a student, "well, that's not what I meant!" Unless mind reading is criteria in your psychic examination, it is inappropriate to hold students accountable for the instructor's intentions. If they got the literal answer to one's poorly worded question or followed one's poorly worded instructions literally, they get the mark or behaviour.
Second, you have to love this student! That is out of the box thinking! Exactly what every discipline needs more of.
Third, open book examinations are actually harder. Okay, if it's closed book for everyone else and open book for you, that does give one a slight advantage, but not as much as most people assume. There is relatively little knowledge that we need to memorize, as opposed to understanding and having available to us on our computers or google or etc. In most disciplines, one ends up memorize the important facts because that's easier than looking them up every time. If the fact is one you have to look up, it is by definition one that did not come up enough to be worth memorizing. Exams should be testing understanding, not just rote memorization. If the exam is testing higher orders of cognition (analysis, synthesis, evaluation and so on) then making information available to students makes for a better test of skills than the usual rote memorization that many exams seem to be geared towards.