When looking at the geological record for a given locality we have to look at extraneous factors factors which might influence conditions. There is a Croll quote from Lyell's Antiquity of Man referring to a warmer Greenland in the past and evidence of plant life there. However, Greenland may be atypical because of the presence of "numerous hot springs" on Disko Island, the location mentioned. This may be a contributing factor for Sequoia langsdorfii being able to grow at such a high latitude.
It would be interesting to see what radiocarbon or some other form of radiometric dating would tell us about the age of the fossils found. One has to be careful about the assumptions one makes when extrapolating the proxy temperature records and not read too much into them. Paleontology and the fossil record may provide some additional information over a longer span of time.