D&D 5E Clarification on a Marilith's "Reactive" ability

[Kithas]

Yep:
To quote


It is admittedly a strong contender for "most easy to miss rule in 5E", along with the restrictions on casting with bonus action spells.

My vote for that is the specifics of advantage vs disadvantage from multiple sources. The fact that they always null I didn't even notice till like last week.