What word or phrase most concisely describes speaking with wrongly assumed knowledge on behalf of the hearer?
The fancy explainer is talking over your head or the analogy went right past you. Meaning that nothing is sinking in to your brain or your understanding.
To go over the head: to fail to understand something She was being sarcastic, but he took her seriously - the joke totally went over his head.
Cambridge Idioms Dictionary, 2nd ed.
Your explanation is above my pay grade.
The expression originates from the U.S. Military, where the term "pay grade" is synonymous with rank, typically enlisted rank. Those with higher rank are paid more, due to having attained more knowledge and assuming more responsibility.
It is rapidly transferring to the general population as a way of saying what you are describing. Idioms by Free Dictionary has this:
- Above or beyond one's general skill, knowledge, ability, or willingness to participate.
The expression is even being used in an internet meme (warning: link leads to a Google image search with some results NSFW).