I think the idea was removing, “They’re here,” avoids having it look like no one told Maz about the impending attack.
Well, technically, since the approaching TIEs were removed there is no certain attack, and even if they suspect an attack might be coming they have no idea when it’s happening or if they will be attacking the castle itself. So to me, “they’re here” is not about total surprise, but a sort of disbelief and shock that they are now actually HERE, at her home, attacking directly.
It could be removed, that’s something to try, but I feel like it works like it is.