He said “STP” or “standard temperature and pressure”, which controls for the things that affect the measurement changing.

Even if the effects were not properly accounted for in the calibration of thermometers (which I doubt, in general), it would not likely cause a systematic error that shifts slowly over decades. The authors of the report have identified, and explored, other, more plausible souces of such systematic errors, such as in how the measurements are performed.

Do you not own a barometer? Pressure changes by the hour.

Indeed it does, but I cannot see how that could possibly refute the point I am making. How could that cause a systematic error that shifts slowly over decades?

FWIW quartz thermometers achieved accuracies better than 0.05 K in the 60s, with resolution better than 1 mK.

Boiling water is at 100C at standard pressure full stop. It is not affected by weather (humidity or air temperature). It is not affected by location unless your change of location involves a change in altitude which accompanying pressure change.

Air pressure changes with weather. It's easily visible in old school barometers like http://www.4physics.com/phy_demo/pressure/baro_exp1.jpg

It varies in a range of about +5% to -5% of average pressure.

Which was known about at the time. The ideal gas law for example dates back to 1834 or 27 years before the oldest measurements in this dataset. But even that was not needed with Barometer’s dating back to 1643 and vacuum pumps originating in 1650 allowed for pressure calibration making the current weather unimportant.

Anyway, I think people are vastly underestimating just how far scientists where calibrating things back then. Clock making for example cares deeply about the current temperature.