It's not IDB as they seem to be covered by the CoCs here. Where they screwed the pooch, so to speak, on this was in not making it clear to the OP (and I'm assuming that they didn't send or convey adequate information to this effect else the original rant would be totally off-base) that the "reservation" was still subject to first-come-first-served rules.
Cheers.
First come, first serve even w/a reservation is not the set rule for a pet in cabin, I'm wondering however if it is an add'l req unique to the ex Grenada flights.