The
full wording of FCL.905.FI concerning aerobatic instruction is:
(f) a towing, aerobatic or, in the case of an FI(S), a cloud flying rating, provided that such privileges are held and the FI has demonstrated the ability to instruct for that rating to an FI qualified in accordance with point (i);
In other words, to instruct for the Aerobatic Rating you must both hold the rating yourself
and have demonstrated ability to instruct to an FIC instructor who is qualified to make an assessment.