I wasn't 100% sure, but the data backs this up[0]; the "3-2-1 backup rule" is a relatively modern concept, at least by that name. I also remember things like "have one backup in one place, another in another place" since before this mnemonic, but I feel like it's unfair to judge a 70+ years archive on something that just recently actually became a widespread mantra.
- [0] Interestingly, Ngram (https://books.google.com/ngrams/graph?content=%5B3-2-1+backu...) says ~2010 while Trends (https://trends.google.com/explore?q=3-2-1%2520backup%2520rul...) says slow build up from ~2020, guess "everyday" language would sit somewhere around those two different truths.