I think you have that backwards. Snyder is saying that the historical record shows that the sensitivity of temperature to carbon dioxide is much HIGHER than the GISS estimates.
Gavin Schmidt's comment is, basically, that her data shows correlation, not causation.
I took away from her study that, as far as she could extrapolate from the available data on climate/temperature cycles going back 2 million years, that we were pretty much smack at the point of the two curves one would expect during this point in time, so to speak, on both CO2 and temperature, and from that lack of deviation from expected norms then suggesting that humans have had little if any significant effect on global temperature averages, and that the warming that is occurring and will continue for a long time at pretty much the same average rate is pretty well inevitable given past history with or without human industrialization.
Seeing as how industrialization in it's entirety has failed to have been shown to appreciably affect global temperature changes then massive, costly, and punitive CO2 mitigation schemes become pointless and wasteful. The problem being that a non-existent 'climate crisis' allows governments, politicians, and their bureaucracies unprecedented powers and control that they will never willingly give up.
And so the beat goes on...