It actually says the ice shelf broke up about 1,400 years ago ( 600AD) and reformed approximately 800 years ago ( 1200AD.)
Rather coincidental to the Medieval Warm Period
600 AD matches closely the Algonkin and Iroquois legends of mass migrations of war-like peoples from the south to the southern borders of Canada in 700 A.D. where they clashed with Algonkin and were defeated.
It also corresponds to the period when the high civilizations in Chaco canyon and Cahokia flourished. Cahokia was larger than most European cities of the time