Every interviewer seems to have some set of these weird little expectations about behavior, that are basically arbitrary. Worse, some are mutually exclusive (maybe not so much in this case—though, see elsewhere in the thread where someone mentions this seems like a fresh-out-of-code-camp thing, so it might send a negative signal!) so lots of rejections end up having a reason behind them, but a very stupid reason that you can’t really defend against, to be blunt.