The Paladin skill used was definitely casted. You can watch as it uses the same casting animation as Clemency. Considering its effect and its position its clearly some kind of AoE damage skill, but yet possibly viable on single target. Looks very similar to what would happen if PLD obtained Holy. More likely is that Paladin got their own version of Holiest of Holy somewhere between 60 and 70. The question remains how viable it would be in an AoE situation where they risk being interrupted though. Hopefully the team thought this through.