But as these are real notes, they can be uniquely identified by the serial number anyway.
They never get to a part where they explain how this technique is useful in telling real notes from fake, since fake plastic notes would also have unique textures.
>[Our method] ensures that even in the extreme case when counterfeiters have procured the same printing equipment and ink as used by a legitimate government, counterfeiting banknotes remains infeasible because of the difficulty to replicate a stochastic manufacturing process.
Which means each note manufacturer has its own signature, though whether the researchers confirmed that with more than random samples from circulation, I haven't been able to tell. The full paper is here: https://arxiv.org/abs/2103.06184
It takes a while for all the old bills to get out of circulation though -- a few years ago I got my hands on a $20 bill from 1970 and the kid at the corner store almost didn't take it since they had apparently never seen one that old, probably didn't help it was nice and crisp as well. Only thing I can imagine is someone was saving it from their birth year and had to spend it for whatever reason since $20 in 1970 was over $100 in today's dollars so they lost a bunch of value holding that bill.
I do not know how, and how frequently, current anti-counterfeiting measures are checked, but the more subtle of them are almost never checked in ordinary cash transactions. Presumably they are checked somewhere?