Wrong on both points.
In this discussion we are taking the flood as a given.
We are not doing anything of the sort, or at least I am not. You are taking the flood as a given, as it seems is standard practice with you. I and others are trying to show you why that idea doesn't work in the real world.
No doubt, but nevertheless the RESULT of the impact, the effect of debris, dust, steam etc., can't be known with all that certainty. There are other variables in the mix that can affect those results.
The effects of such an impact in any possible environmental conditions can be easily extrapolated with existing knowledge. With the largest, it really is utter catastrophe.
The tired old claim that "We didn't see it so we don't know, can't know and can't reasonably assert what happened," is a ridiculous claim. A citizen of Tokyo holds a pencil out in front of there face and opens their hand so that the pencil is unsupported. We can assert that the pencil will fall. If we know the brand of the pencil we can estimate it's weight. If we know how far it falls before impact we can (even if only by estimating the average height of a citizen of Tokyo) how fast it was moving when it hit (within a reasonable range) and with that information we can determine likely effects of that impact on various surfaces. It won't be a whole lot, of course.
No one saw this event except for our Japanese friend (and let's assume he isn't going to talk) so we have no eyewitness information, yet we can make all sorts of good determinations about it.
The same is true with massive cometary impacts, only the numbers are a good deal larger.