The real reason for the cancellation is that the airplane never left Madrid. Therefore there was no airplane available to fly FRA-MAD.

It's irrelevant to get into an email argument with IB over why they cancelled your flight - you won't win that argument. It's also not your obligation to prove to AESA / a court that the reason for the cancellation is covered by EC261 - it's Iberia's obligation to prove that it isn't.

Pilots are rarely given the go-ahead for pullback only to be told by ATC that they aren't allowed to take off after all. (it would only be something like the freak runway closure like recently happened at LGW or a passenger suddenly became ill) It's more likely that the pilots didn't like something or other with the aircraft and went back to gate. (and that wouldn't be extraordinary for EC261 purposes) So it's worth your effort IMO to claim from IB and then go to AESA or elsewhere if IB deny your claim.