Earlier quoted context omitted.
I thought that it still isn’t really known? Is is one common theory, but we have just probabilities to play with.
Finding the impact crater pretty much cemented it. It absolutely happened. The remaining questions are around if the impact was enough to trigger the extinction on its own or if other factors compounded the problem.
Well, in the sense that we can never be sure of anything, yeah. But it not being enough is a really extraordinary idea.