You presumably don't know that the HCO period of warming correlates with the periodic changes in axial tilt. The Northern Hemisphere warmed as you state, and the Southern Hemisphere cooled.

It wasn't a global warming, but an increase in solar to the North. Globally temperatures were probably similar to today. Any runaway effects in the North and Arctic would have been compensated by increased glaciation in the South.

That's why Antarctic cores go back further - it didn't melt like the Arctic did.