I'll talk to my friends about "my day" every once in a while. Invariably it involves programming and the act of trying to describe what I'm doing is like off-roading: you constantly have to go long ways around things to avoid crossings the conversation can't handle, it's rough and bumpy the whole way, and some destinations simply can not be reached.
In short: I agree that rambling may have more to do with the difficulty of translation than the internal understanding.
I find that you can often use analogies or describe very vaguely to at least make some sense. It does take nuance out of the thing you’re describing but maybe that’s helpful. I’ve had multiple times that I explain in simpler terms, and I come to realize what my idea actually looks like to other people.
Some people bristle at analogies for the highly technical things they do. I think they misunderstand the purpose, or think it mocks the work they've put into understanding the subject. A good analogy creates a bridge toward interest, and interest leads to the focus needed to learn the technical stuff.
In short: I agree that rambling may have more to do with the difficulty of translation than the internal understanding.