The boss has legendary resistances, so it has a set number of times where if it fails a saving throw to avoid a spell or other effect (such as banishment), it can choose to succeed instead. Divination wizards have the portent ability, which allows them to replace a roll by anyone they can see with a prerolled die from earlier in the day. Legendary resistances make this irrelevant for saving throws, since the boss can just pass regardless of the roll, but initiative to determine turn order is not a saving throw so it can't use legendary resistances.
I got confused and thought big guy was calling new guy a noob and downvoting him.