Their note does place it into one of the major ice sheet coverings, I think.
I've read that cosmic impacts on land causes it to get colder while an impact in water causes things to warm up. I think impacting into ice would be similar to results of a water impact, warming.
BTW, some pretty respectable scientists think a comet or comet fragment impacts caused the last Ice Age to end and we're still enjoying the benefits from that.
This chart is an indication of temperature changes over the last one million years. The article is about an event 60-65 million years ago. This chart has nothing to do with that time period. Also, any evidence of tsunami wash would have long ago been destroyed by many, many ice sheets.