Can't speak for the writer themselves but the reason can be that Arcane Comet is super easy to dodge. Viktor doesn't have much CC on his damaging abilities to make the comet easy to successfuly land making Summon Aery much more reliable way to poke off his targets while giving him the awesomness of Sorcery tree. Also Summon Aery is procable via auto attacks while Arcane Comet isn't, which can be pretty meaningful difference in the game especially against mellee mid laners that can be auto attacked easily.

thank you!