Yes, I think the comment was a bit hard on the instructor. Remember, an instructor is not an engineer. The answer was probably got by people on this site, but I bet it took over a second for them to work it out.
If something like this happens your brain should immediately be moving towards the memory items checklist. You don't have brain time to diagnose things until you have got those items sorted, and then only if you have time, which at 300ft AGL is not going to happen. I also think it been shown it takes about five seconds for the brain to actually realise something is wrong and start to do something about it, and that is during check outs where the pilot is expecting something.