who said this was fast? Maybe it took him weeks/months to get so much.
Also, i know players who keep every interesting pokemon to transfer for candy in a double candy event. Spoke to someone who had 100 torchiks ready to transfer but was bummed the event didn’t involve double candy. Do that for multiple mons and before you know it: full box.
Also, keeping mons for evolve spree during xp event fills up your box pretty fast.