I'm guessing they were refering to Nooks trading codes in all games preceeding Wild World. I think each region and separate game in Japan had different Universal Codes, and I'm pretty sure Japanese codes didn't work for the rest of the world, but I think everywhere else had 1 set. (I think because Japan skipped over our Animal Crossing for Gamecube and went straight to e+, so they were never compatable)
But Wild World and City Folk were both completely compatable.
I always wondered "What if the QR codes I've been saving up were region locked"
Just like, hypothetical.
But at least this clears it up! So that there isn't any doubts.