They might think this because they measure toxicity based upon reports. If ppl queue up together more often than in the previous system, then yes, reports as a whole will most likely go down, because ppl that queue up together will most likely not report each other. It may lead to this kind of "gang up on somebody" bullying that while limited to one player in the past, is now extended to whatever size the premade is. TBH, though, I don't see a lot of premades on my teams as i am a solo player, but I could easily see this kind of behavior existing in bronze and silver. Riot hoped to decrease toxicity by implementing dynamic queue, but some players have seen premade bullying occur.