First question - Does the text of the letter ask for a reply? If not, ignore it.

If you need to reply, simply thank them for their letter, state that all flights of this aircraft are aerobatic, and state that you understand that the ADS-B equipment only works in level flight. You can state that FAA is aware of this and that you understand that the International Aerobatic Club is speaking with the agency on working on this deficiency in their system.

Best of