It's not really something that is seen as needing proof, I just heard that occasionally NiKo was toxic, I never questioned it. If these claims are indeed incorrect, then fair enough. Otherwise, it isn't something I've tried to debunk because I didn't think it was necessary.
CS:GO pros rage from time to time, there is nothing wrong with it unless the raging is out of control and becomes a detriment to the players/game itself but we all have that niggling in the back of our minds, being slightly annoyed here and there.
Toxicity is misconstrued and that is more apparent on HLTV than anywhere else, so take it with a pinch of salt, friend.