Well, the first reason that is silly is that the impeachment inquiry exists to find out what charges, if any, should be brought.
The second reason it is silly is that the requests from the committee have been very specific:
"related to reported efforts by President Trump and his associates to improperly pressure the Ukrainian government to assist the President's bid for reelection."
The third reason is that Pompeii has no such rights to demand any of that. His refusal is only provided cover by the fact that the DOJ will not prosecute him for defiance, and that the Congress is not yet to the point of invoking their powers regarding inherent contempt.
Given that the second reason is not founded or grounded in any fact, that would require much more clarification from the Congress. A good reason to do as I said.
He has every right to refuse an improper subpoena.
The impeachment farce should already have an agenda with a detailed listing of reports from Justice of an independent investigation listing the concerns and wrong doings of the President in order to justify the impeachment. They have none of that and Pompeo is under no obligation to aid them in their witch hunt
Congress is not above the Executive branch.