Interdasting anon. Unless Q chimes in, I would suggest looking at the exact wording of your question.
You might actually need the names of the D's who wrote the letter, or even the exact combination of the names of all six, for example (but hopefully not).
It's a bureaucracy after all, they like to stick to the exact procedure etc. They didn't say the letter didn't exist, just that "no documents responded to your request".
But good on you for requesting that! If you try again, maybe take a guess at which senior D's might have written the letter, then file six separate FOIAs, for any correspondence involving D[1] to Don McGahn between 01 Oct 2018 and 15 Oct 2018 regarding Hatch Act Violations, D[2] to Don McGahn between 10/1/18… etc.