insular

I have been teaching English on italki for about a month now. I have had satisifed students, but recently I had a lesson with a Jaoanese man, who left feedback saying that the lesson was good, but who then wrote me a private message to say that I had been rude to him and the Japanese in the lesson.

I had asked the man if he thought it fair to describe the Japanese as an island people, which, literally speaking, they are. I used the adjective 'insular' as well to try to improve his vocabulary. During the lesson, he gave me no hint that he was annoyed by my use of that adjective. In the feedback, it looked as though he was satisfied with me. However, then came the private message. He said that since he was paying for the lesson, it was very rude of me to use the word 'insular' to describe the Japanese. I immediately wrote back and apologised for my unintentional 'rudeness'. However, for the life of me, I cannot understand how the word 'insular' can be deemed to be 'very rude'. Furthermore, I did not even say that the Japanese were insular, I asked him for his opinion: I asked him if he thought it was fair to describe the Japanese as 'insular'. I do not see in any of the dictionaries that 'insular' is a pejorative adjective. Furthermore I googled "Japan' and 'insular' and I got thousands of hits, some of which were from highly reputable newspapers [NY Times and the Japan Times]. I was wondering if anybody has had similar experiences and whether this is something that I should have to get used to. Furthermore, if there are any native Japanese speakers here, could they please let me know, if I committed a major faux pas, because to tell you the truth I am rather distressed about this. I had no intention whatsoever to imply or insinuate anything bad about Japan or the Japanese, about whom I know very little. Is this word something that raises Japanese hackles or is it just a personal quirk of one particular Japanese student? 

I would be deeply grateful for any input, particularly from native Japanese speakers or from people who have experience of living and working in Japan. Thank you.