Under Catch Em All for Emerald, there’s a category to complete the Hoenn Dex using a single cartridge, which as far as I know is impossible. Should this be changed, or is there something I’m missing?

Great question. I wonder if it's supposed to mean all the Hoenn Dex captures that are possible in only one cartridge? Kinda like a Professor Oak's Challenge

So this was originally supposed to be a Glitched Category. The goal was to collect the 200 Pokemon in the Hoenn Dex (possibly 202 since we could also glitch in Deoxys and Jirachi) using the Pomeg Berry/Glitzer Pop glitch. If you want the "Professor Oak's Challenge", it's the 212 Catches (Glitchless) category.