Thr problem is that not only do we not know if this actually was an impact crater, we also have no strong connection between it and the time period in question. Much of the supposed evidence of an impact is highly disputed as well, such as claims about "black mats" and "nanodiamonds." From what I have read on it, much of the support for the theory is being pushed by a small minority. The evidence is classic psuedoscience in that it really relies on connections that are far from concrete. When the evidence is inconclusive, a researcher may say it is possible for certain outcomes to cause it. These pseudoscientists then take that to mean it may have been an impact, and cite it as "proof" for their theory. The issue is that if all your evidence is not concrete, then you really have nothing supporting your theory. Basically they choose what to use as evidence and what to ignore based on how well it fits the theory.

The real answer is that until they can prove this theory, we have no reason to believe a massive impact occured so recently. There is no consensus in the archaeological community that such an event occured, and thus trying to speculate how it impacted the world is a bit like trying to speculate how aliens shaped WW2 or how Bigfoot influenced US foreign policy during the Civil War.