I haven't played BG3 but I've played 5e to death.

She presumably has a single 9th level spell slot, as that's the most almost any creature can get. Wish is 9th level. Wish can replicate any lower level spell regardless of if the caster knows it or could cast it, or you can say fuck it and do basically anything, with a 1/3 chance of losing it to cast forever.

There are a few ways to just annihilate an adventuring party of lower level, such as casting fireball 9th level, but it sounds like she thought she'd risk the hail Mary and actually wished the party to die without a spell.

It seems that alternatively our end up in the astral plane and she uses wish to cast something like scrying which she didn't have prepared, meaning she couldn't then use the spell slot to wish the party to death.