Guys, my inquiry was not about "extraordinary circumstances" and compensation under EU Reg. 261/04 nor about the duty of care.
Obviously, the air spare closure is an extraordinary circumstance exempting the carriers from paying compensation regardless when the ticket was purchased (to the extent of course that the delay was only caused by the air space closure but it may turn out difficult to prove that it was caused by something else) but AI still has a duty of care regardless of the reason for the delay. It seems that AI fulfills the duty of care obligations as could reasonably be expected.