It's entirely plausible to me that this has gone unnoticed for 200 years.
Two possibilities come to mind. First, he may have been studying a rare brain that had an enlarged system, and since he did not have preconceptions that what he found couldn't possibly be a component of the lymphatic system, he called it like he saw it. (It is also possible such brains were less rare back then or something. Many things have changed in our body's environments in the last 200 years. Random correlations abound.)
The other is that perhaps he was simply wrong. He saw something that he identified as a lymphatic component but was in fact something else. Future people didn't make the same mistake so the observation sank without a trace. 200 years later, the real and much-more-subtle lymphatic system was found.
[1] http://news.stanford.edu/news/2014/november/mystery-brain-im...
Video description:
Leonardo became obsessed with the heart in the latter period of his remarkable life. He made a prediction about the main blood vessel leaving the heart, the aorta. He had figured out how the flow of blood out of the heart actually closes the aortic valve.
Leonardo had no way of testing it and no way of seeing inside a working heart. For centuries no one else even speculated. But in the last 50 years technology has finally been invented to show that he was absolutely right.