I think luck becomes less of an issue than good problem-solving/critical-thinking skills because the passages are made in such a way that you *should* be able to solve them with minimal prior knowledge (i.e., just your basic science stuff). A person might luck out on one or two topics that happen to be familiar to them thereby circumventing the need to apply the fundamentals in novel ways (i.e., problem solve). However, it's highly unlikely that top-scorers have acquired detailed knowledge in nearly all topics. Chances are (luck notwithstanding) that they've scored well by exhibiting the very skills the MCAT is trying to test (e.g., critical thinking, etc.).
That is to say, if you hit 40 luck had a smaller part to play than did your actual skills. Conversely, if you hit a composite score of 12 then it's all you 'cause the answers were there for the taking luck or no luck. Does luck play a part? Yeah, sure, such is life. But I doubt luck alone (good, bad or ugly) would be enough to put someone in an entirely different score range. 😳