My first-year philsophy professor gave a similar warning. He offered words to the effect that if we (the students) started thinking that the philosophy we were reading made no sense, was idiocy, or just nonsense, pause for a moment and imagine whether we thought it was likely that Plato was an idiot and everyone hasd missed it for millenia, or that we might be misinterpeting something. There’s always the chance that we’ll have an insight that others might not have had, but there’s a reasonable chance the the philosophers we were to study might not be exclusively stupid.
I think it's a question of internalization. When you have internalized the correct way to think about it, it feels obvious. And wrong ways to think about it seem absurd. We stand on the shoulders of giants, Plato being one of them. We have internalized their philosophy. And that's why it feels idiotic.
That's a good professor. I think everyone who has dabbled in Philosophy, especially of the ancients, usually Plato, has had that experience of thinking they are smarter than Plato and that he is making obvious errors all over the place. The first thing any student of philosophy (or a lot of other fields, really) has to do is to cultivate epistemic humility; he must train his instinctive reaction to "I don't get it" or "this is obvious" away from "therefore the author is dumb" and towards "I must be missing something".
That isn't to say that every author and philosopher is infallible and you must assume they are smarter than you at all times, but just that you need to acknowledge that very famous works are typically very famous because, in the opinion of very learned people, they contain depths to be plumbed. If one isn't seeing those depths, he needs to look harder.
Plato wasn't an idiot.
But generations of philosophers who have described the next couple of thousand years as "footnotes to Plato", and treat philosophy as if it can only be understood in terms of one very dead guy... It turned me off philosophy for a very long time. Maybe they aren't stupid, but they at least lack the tool kit for showing me the merit of their position.