Climate is Cyclic. The Earth Rotates and orbits around the Sun. Just those two create 24 hour and 365.26 day cycles. Since the Earth rotation is not perfectly smooth, it slowly wobbles around with a period of roughly 21,000 years, called precession. There are more, but just consider the annual and 21,000 year cycles.
The Annual cycle causes Earth to have seasons. The 21,000 year cycle causes the seasons to swap. Right now, since the Southern Hemisphere is in the 21,000 year "summer" period, it would be warming just like the Southern Hemisphere is warmer in December through February, their Summer, during the annual cycle.
Marcott et al. averaged the Holocene, a period of roughly 11,000 years or half a precession cycle.
Marcott et al. were only looking for "average", their method would suppress any synchronization of hemisphere impacts.
Since the precessional cycle is roughly 21,000 years long, each precessional "season" would be 5250 years. Since we are used to thinking in months, each precessional month would be 1750 years long. Again comparing to the calendar, each precessional day would be 58.33 years long.
So Marcott et al. is essentially saying that it is unprecedented for the last precessional day, 58.33 years, to be warmer that the average of a precessional winter and spring.
We actually pay these guys to do this stuff.