I personally interpret your story not as evidence that Lemmy is insular. Or at least not in the way that perhaps you intended it. It seems to me (and this has generally been by experience with Reddit) that Reddit is generally really good at putting people together with others of a similar viewpoint. To me, the fact that you are more accepted on Reddit seems more indicative of the fact that Reddit prevents people who disagree from even talking to each other. Downvotes and upvotes, after all, have basically never been used as a measure of discussion. Both here and on Reddit, they just measure how many people agree with you.
My experience on Lemmy has generally been that even while people disagree with you, they make a more earnest attempt to engage with your viewpoint.