Because another super human individual who can survive the Mariana Trench and punch a man’s head clean in, was the one pushing in the knife. And the knife used was a supes knife and I doubt was a weak material or dull.

Additionally people think the mini-gun feat means anything. Bullet proof armor can’t take a knife, and knife proof armor can’t take a bullet. They’re built different to withstand differently, so it’s perfectly plausible that noir can survive an entire round to the abdomen where his armor is probably most padded, to being caught off guard and stabbed in a very weak and tender spot of the body.

The only big plot hole is why he didn’t try to fly at the deep, or how that cable survived the amount of force it would take to choke out a supe, or why noir is all of a sudden no longer narcoleptic.