I guess their assumption is anything cast right away will have Legendary Resistance used if the save is failed and the spell is sufficiently debilitating, so better to use Polymorph since it's lower level but strong enough to warrant using LR.

Disintegrate does x damage. You know what does more than x damage? 3 other characters holding their attack and spell actions after you bap the lizard sitting in a cage, after killing all of their henchmen. But they’ll resist it anyway, so it’s effectively less magic spent for the same result

I'll use baleful polymorph