I think the whole point of this scenario is to prove they didn't do a very good job building the monsters in this edition. A high level monster should be able to deal with flying enemeis with abilities in his stat block.
Jump ability, sonic scream, throw debris ability, calls down Gamera, pretty much anything at all to deal with a ranged attacker.
It is almost like they didn't learn anything from the first few decades of gaming.
No, they learned it all too well. They learned that no matter what they put in a statblock, armchair debaters would find all manner of "holes" in it and create all manner of scenarios for which supposedly outgunned creatures could defeat more powerful creatures by following the "letter of the rules".
So WotC has just said "F-that". They made a book full of really good monsters and put it upon DMs to run them fairly and in the manner they wanted... making up whatever they needed for compelling fights and drama. The good old "Rulings, not rules" rearing it's wonderful head again. And if that drives certain DMs up a wall because they don't want to make logical rulings and instead stick with hammered-out statblock rules... then WotC seems to be okay with that. As am I. And I hope against hope WotC never gives in to these armchair debaters, going back to try and "patch in" all these supposed holes, because even after they did, the armchair debaters would just move on to another ridiculous theoretical scenario that would never actually come up during play.