If I thought you knew it, I wouldn't say it. I don't see why that's offensive - I can see why you wouldn't like it, but your comments make it sound like you've never handled this kind of criticism before. Prefacing my statement with "surely" or "as I'm sure you know" would frankly be disingenuous, because I certainly did think you had not dealt with academic-style peer review.

This style of criticism - no frills, no pleasantries, not even necessarily any suggestions for improvements, just "this is what you did wrong" - is common when reviewing research. I, in fact, have five such reviews of my own work I need to rebut.