Board Thread:Naruto Discussions/@comment-24048522-20191229052502/@comment-1210805-20191230132726

No idea. Kaguya isn't exactly the most sane character we've seen, and as for how well she understood her or anyone else's powers is up for debate. She only ever fought her sons before, both of whom had senjutsu, so maybe she didn't know that she was resistant to regular ninjutsu. In reality I think it's likely that Kishimoto made an oversight in that chapter and it was carried over to the anime.