I don't know about that, from the context (and the description), it seems far more likely to apply to Orcus. Given that I imagine Bahamut's Dragon Form would involve a change in size, flight, breath weapon, and much more than a slight modifier to stats, resistance, and the addition of an attack that would compensate for a known weakness of Orcus.
Of course, I am certainly eager to see Bahamut's dragon form anyway! If it is nearly as awesome as his "Old Man" stats, I'll certainly be happy. (And I am more than glad to have thematically fitting but nonetheless easy ways for great wyrms to engage in such an epic fight, without simply throwing thousands of hp in allies into the fray, and letting slavish devotion to formula and simulationism doom the fight to boredom and grind!) The Old Man with the Canaries is a downright inspired piece of game design, and if the use of the great wyrms comes across as items rather than real creatures, I think the fault would fall with the DM rather than the rules.
