He knows the plot... but he's a speedrunner. So, there's a certain difference between knowing and "knowing" here. Regardless, we have seen he doesn't spend any time at all pondering the story he supposedly knows. We have got exactly zero scenes of Len trying to think of the reasons the original Len became a villain. I mentioned it the comments of an earlier chapter, but since he doesn't really care about the story at all, he's not preparing himself desperately either. He spent great many years tilling the land, instead of fighting lesser monsters. He believes he can keep living in that village forever, all happily with his family, despite "knowing" the original Len most certainly couldn't. I bet he's a natural speedrunner, not just someone doing it for the sake of Youtube clicks, because he kept skipping dialogue and never read any in-game books or other sources of information, probably finding it deadly boring. So, I'd assume he doesn't really know anything but the most important plot points.