Its a difficult question. On one hand, AFOLs are constantly borrowing techniques from each other. So much so that the tudor-style has become something of a standard. The employment of SNOT to create archer loops, or elaborate tudoring has also been employed by countless builders. Still, details alone do not define a MOC.
I think it is more offensive when somebody's borrowing goes beyond mere detailing. This is more common with smaller MOCs. If you are so bold as to essentially plagiarize someone without giving credit, that person would be in his or her right to be completely offended. A perfect example would be some of the small furnishings that builders like Jojo build. His furnishings are incredibly unique. In most cases, a copy would be easy to identify.
Still, this problem can be further complicated when a design is continuously plagairized, making it difficult for others to identify the original MOCer. For example, many MOCers now incorporate similar beds and cabinets into their MOCs, without recognizing the original designer. How many builders actually know who made the first bed using

. I've seen countless pieces of furniture that use jumper bricks for drawers, yet I couldn't say with any certainty who was the first to do this. Still, builders don't want their MOCs to look inferior when a technique becomes commonplace.
Your situation is different. This is not incidental. Nor is it simply a builder borrowing one of your techniques. This is a blatant copy of one of your MOCs; a MOC that is unique to you, and you alone. It would be like copying Jojo's furniture and passing it off as your own.
What I find most offensive is the fact that the guilty party acknowledges that he plagiarized you, but then purposely downplays his crime. Essentially, he admits his theft, but trys to make it seem inconsequential. It is plainly obvious that he used the images in your gallery to construct his MOC. To say that he doesn't know who the original builder is, is a flat out lie.
email him.
Later.