This is ahistorical. The original Lemmy instance is lemmy.ml, and it was hugely tankie literally from the beginning - the .ml referring to marxist-leninism, years before Reddit’s API changes. It’s nothing to do with people being banned from Reddit, it’s just that the concept of a federated message board platform was appealing to communist software developers, who created and guided the project. If anything, the anti-tankie sentiment which is popular on instances like lemmy.world is what came to lemmy after the Reddit exodus.