I've also seen many other embellishments mentioning Greenland, the Vikings, Vineyards in York, frost fairs on the Thames etc. Also I've emailed over the years for the numbers in the 1990 IPCC Figure. I even got a digitized version once from Richard Tol and told him what he'd done was ludicrous.
If I read that correctly in context, they were complaining that their temperature reconstructions from proxies were being nit-picked by people who said that it was hotter or colder during certain periods than their proxies implied.
Examples - the Thames freezing over in the 1700s and fairs being held on the ice. Greenland being warm enough to support a large colony of Vikings who farmed a variety of crops for hundreds of years.
My take on this snippet is that these scientists are so taken with their elegant reconstructions from tree rings, peat bogs, ice cores and so on that they can't believe that maybe they are not as accurate as they think they are.
So if their tree ring data says that say a particular decade was not that cold, but painters in Europe painted freezing winters and diarists wrote about freezing winters and financial records show poor crops etc, then they are inclined to dismiss the painters, writers and record keepers as bunk as their tree rings show the "truth".
If I have that right, it shows a remarkably closed mindset, and an unhealthy addiction to certain tools and techniques. Then again, if your job depends on your tree ring work, I guess you will apply it to everything and defend it to the death.