I think you're taking that as a personal insult, when I don't think it's really meant to be. He was just saying that mainstream Christian sects, including Catholicism, vary mostly in interpretation of the same core belief system. Mormonism, while certainly Christian by the most basic definition, has more significant differences from mainstream Christianity, which reach beyond simple interpretation. Personally the example I'd use is Judaism and Islam; while both are based on the same interpretation of events, and neither believes that Jesus is the Messiah, Islam believes in an additional prophet and an additional set of religious laws based on his words. Mormonism differs from mainstream Christianity in much the same way. So saying it's different from Christianity isn't entirely correct (because Christianity in essence just means belief in Jesus as the Messiah), it's aiming at something correct. There's just not a great way to articulate it, since there's no word I know of for "Christian, but without additional theological additions".