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.
The clear implication of the earlier statement in context is that lack of anonymity should be expected to reduce the practical aspects of the problem to a useful degree. I believe the general state of facebook stands as a stark counterexample.
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?
Yes, it is far worse because it gets personal far more quickly with non-anonymous users. People don't feel exhausted by 4chan, it is mostly the comparison to others on "legit" platforms.
And to be frank, there is value in 4chan. Nobody would feel compelled to believe obvious bullshit some societies and cultures impose on its members. This shit needs a counterweight. That isn't 4chan of course, but any free platform.