Does that mean Newton didn't understand his stuff? Or that he wasn't a first class scientists? I don't think so.
Yes, I think it's indicative of not being a first-class scientist. I don't think anyone would say that Newton wasn't brilliant, but part of being a scientist is making Knowledge available to the world. The pursuit of science is (or should be) ultimately altruistic, and if Newton deliberately made his writings harder to read than they could have been, then he was a second-rate scientist. He allowed his ego to get in the way of the real purpose of scientific exploration.