i dont think khazix, cassiopea, kogmaw or alistar are considered op by anybody right now. your enemy team just straight out had a very bad teamcomp with little to no cc other than alistar and only single target focused while all dmg are extremely squishy and easy to be killed.
i can see how the teamfight went, gnar or amumu goes in for some cc, and mf ult, kogmaw/khazix/cassiopea got deleted and garen/alistar run for their life but still got chased down.
they arent blindly picking op champ, they just have a bad teamcomp.