Hi Tarzan,
Unfortunately, the datasheet for the AD620 was written over 20 years ago so we do not have a direct resource to provide you with this information. Most likely, it came from the characterization data of the OP07D. A resource you can use for calculating nonlinearity for op amps is MT-044. Take note, however, that because 3 op amps are used for the "home brew" in amp, the linearity cannot be added directly. The errors from the op amps may interact differently - they may cancel, add together - the results would most likely differ. It is possible that the nonlinearity error on the AD620 datasheet may have been calculated using an equation such as the one in MT-044 with the assumption that the errors were equal. However, it is also likely that a direct measurement was taken from the overall discrete 3 op amp in-amp.
I apologize that I cannot give you a more concrete answer given the age of the datasheet. Out of curiosity though, are you considering making a discrete 3 op amp in amp? What specification do you require from your in amp? We have a lot of in amps in our portfolio and I believe we could find one that suits your requirements. Let me know what you need for your application and I can try to find one for you.
Thanks!
Kris