I'd wager the amount on FB is orders of magnitude _more_ than on 4chan. 4chan's average severity is likely higher, though. They're different problems, but saying that online toxicity only exists on anonymous sites is naive at best.
They didn't say that, they said "anonymity leads to lack of accountability". Even the counter that authenticated sites have users engaging in toxic behavior doesn't contradict the original point.
That said, I do think it's a fruitful line of inquiry to wonder why that might be the case. Are there any known cases where anonymity alone is sufficient to tip the balance towards widespread toxic behavior? Related to that, if lack of anonymity frequently does not curtail toxicity then what might the important differences be versus real life interactions that can account for it?