Seems this guy collected data from many different places that had the same results. Also, his time date is about the time that history pertaining to man started. All of this fits into a time frame of 500 years or so. He could be right.
The question is how quick did it take to get cold and how long did it take to get warm and of course which is the norm. The cold or the warm.
When North America was settled by the Europeans we had horrible Winters. It would snow waist deep in Virginia and so on. Prior to that, the Vikings could plant crops in places like Greenland. It is clear there can be drastic climate changes but the question is what brings them on. Meteors strikes would be my best guess. What we do to the climate with man induced events are like trying to bail the ocean out with a bucket.
I don't personally think we can prevent something like this from happening except maybe locally in a city or something. What we have to do is make plans to survive something like this and carry on with our lives.
I agree with pretty much everything you wrote. Good points.