Yes, that is correct. Nearly all the reported warming in the USHCN data base, which is used for nearly all global warming studies and models, is from human-added fudge factors, guesstimates, and corrections.
I know what you are thinking – this is some weird skeptic’s urban legend. Well, actually it comes right from the NOAA web page which describes how they maintain the USHCN data set. Below is the key chart from that site showing the sum of all the plug factors and corrections they add to the raw USHCN measurements: