Technically, Toxicity is down.
People in Dynamic who are toxic normally have friends who are toxic. Friends who are toxic see nothing wrong with being Toxic, or even see it as funny, so they're likely to group and be premades with each other. So now you have a toxic premade of, let's say, 4 people. If all 4 of those toxic players used to be solo players, each of them would be ruining 4 games total. If they were duo players, they were ruining 2 games total. But now, they're just ruining one game total.
So, technically, Riot isn't wrong with toxicity being down. Less games are being ruined by toxic folk, because they're grouping up, meaning they only ruin one game, it's just quite a bit more hostile for the one player.