I don't know that I agree: you're essentially advocating dishonesty, in my opinion. If he legitimately feels that a Unix shop by nature will espouse certain qualities, why should he hold back?
They solicited his feedback. Unless he wants to hold them in his pocket as a fall-back, I can't see why he should sugarcoat his response. Yes, you could call his opinion essentially hubris if you disagree and wanted to be antagonistic, but it's at least honest, straightforward, and not unduly confrontational.
I say this as someone who's been off-and-on involved with interviewing and sent out this kind of question to people who turned us down - the only two times I got a response, they were devoid of any information and just platitudes to butter us up. Literally not a single negative point about us or our process, which needless to say was not helpful and personally annoyed me far more than a letter like this would have done.