TH does not affect it and the range on meteor is 30' from the target, not 25'. That doesn't even matter though, assuming your DD doesn't suck, because you can easily interrupt every meteor he casts. Addle away.
TH doesnt effect drop rate you're right, he probably knows that he is just pointing out what th lvls were on the kills to further highlight theres no point slowing the kill down for higher th. Just get a monk a whm and kill it in 2mins. Yes both KB and KS99 can easily be interrupted via melee dmg its not uncommon for monk at 99 to go entire fight on both ks99 and KB and interrupt all Meteor except the final unavoiable one on the KS99.
Doesnt mean he did it for the defending ring though could be for the abjurations? lol I see no other reason to slow kill down to get th12 for anyone else going into this fight for the ring, dont bother.
Just did two KB runs last night. As it has been noted, it was a pushover fight with my MNK and my wife's WHM. No fancy weapons or anything either. When KB would prep meteor, she'd run out of range and then come back in so she'd just have to heal me and not both.