This is a special case of the last point.
Einstein is said to have said that you have not really understood something until you can simply explain it.
He is also quoted as saying:
"You haven't really understood denmark telegram screening something until you can explain it to your grandmother." ( Source )
No matter what exactly he said, I agree with his statement.
When a supposed expert says or writes the above, there are only two explanations:
He didn't understand "his" topic. He is not an expert.
He has understood his topic. He doesn't want to explain it.
A third explanation would be that he has mutated into a specialist idiot and can no longer provide a comprehensible explanation.
I believe that the “expert” actually could but does not want to.
A special form including insult is: “That is too complex, you can’t understand it.”
Such a claim means the opposite. In truth, it contains two statements:
"You're too stupid to understand that."
"You're not worth me explaining this to you."
This is about knowledge of power that is intended to consolidate relationships.