To be fair, whilst I agree with you from a technical standpoint, there are ways and means of phrasing yourself when you don't think that people are on the right track, and the OP did come across as very dismissive. He may well be right, but being polite rather than confrontational is a better way to get people to help you. I'd have probably replied with something like "Hmmm, not sure that's the issue because X, Y and Z, how did you come to the conclusion that was the problem? What have I missed?", or something along those lines.
You can always call them a **** after you log off and have got the info you need