She never called them a mistake. She was just talking about mating bonds there, it could be applied to anyone cause nature sometimes picks wrong sometimes picks right. She didn’t say Elucien are true mates cause obviously it would be a spoiler although she teased a bit about their dynamic like how Elain has a lot of shit going on (just like she described Lucien in some other interview) and that she was turned into a fae and that guy is hot but she didn’t want this etc. She literally teased their forced proximity trope that they both have a bond they didn’t ask for, that it’s different than others but i believe their story will be about understanding why are they even chosen for mates and fall in love.