The thermometer was invented in the 1700’s, according the Wikipedia. The idea of Duke researchers basing their study on “1,000 years of temperature records” is unsupportable. Further, civilization on any level capable of keeping temperature records did not exist in most parts of the world 1,000 years ago, and not until the last few hundred years in many places...including much of the USA.
1592, Galileo
There are other methods of ESTIMATING pre-thermometer temperatures that give repeatable values. Dendroclimatology uses tree ring widths to get seasonal growing values by locations and ice field banding gives data on snow-fall and winter temps over long time frames.
Obviously not our current standards but it is what can be roughly quantified and thus show trends at the very least.