Authors of the Marcott et. al. (2013)  paper that the XKCD chart uses state:
Q: "Is the rate of global temperature rise over the last 100 years faster than at any time during the past 11,300 years?"
A: "Our study did not directly address this question because the paleotemperature records used in our study have a temporal resolution of ~120 years on average, which precludes us from examining variations in rates of change occurring within a century. Other factors also contribute to smoothing the proxy temperature signals contained in many of the records we used, such as organisms burrowing through deep-sea mud, and chronological uncertainties in the proxy records that tend to smooth the signals when compositing them into a globally averaged reconstruction. We showed that no temperature variability is preserved in our reconstruction at cycles shorter than 300 years, 50% is preserved at 1000-year time scales, and nearly all is preserved at 2000-year periods and longer. Our Monte-Carlo analysis accounts for these sources of uncertainty to yield a robust (albeit smoothed) global record. Any small “upticks” or “downticks” in temperature that last less than several hundred years in our compilation of paleoclimate data are probably not robust, as stated in the paper."
Assuming a resolution period of ~120 years (full cycle) or halfcycle (assuming an effective sample every ~120 years, hence a full cycle period of ~240 years), then the recent excursion can be roughly approximated as a step function. If one lowpasses a step function, you get visible ringing. In the low resolution part of the record there is no ~120 year nor ~240 year ringing at all, so the probability of a similar Heaviside step function theta in temperature having been present but filtered out is nearly zero...
Here's the direct quote again if you missed it above:
"We showed that no temperature variability is preserved in our reconstruction at cycles shorter than 300 years"
At the very least, it seems like cherry picking to focus on 4 degree changes in the last 22,000 years while not explaining 10 degree changes in the last 25,000. I guess 25,000 years wouldn't have fit the narrative so well.
My first thought was "What about the time when a lot of the central US was under water?"
Then I realized the timeline isn't nearly long enough.