Not true at all, Kog was primarily an ADC even before the rework. In fact, he was quite a good one and got nerfs to his base stats as a result. AP Kog was played but far less often and after the nerfs, he was seldom played in competitive. AP Kog remained viable but was nothing special until 35+ min.
Also, it's quite biased to say he "abused his ultimate" when AP Kog gives up so much to make that ult. worthwhile. Weak early, high mana costs, item dependent, extremely high ramping up mana costs on R, needs level 11+ etc. He was literally played only due to the ult. but his kit (low CC, squishy, slow etc.) and build have so many weaknesses that it makes sense to have that be a strength. I don't think it's abuse at all, that would be like saying Tryndamere abuses split-pushing but really, it's just his forte and he's fairly weak in teamfights as a consequence. AP Kog was weak in everything except his ult.