This was his defense back in 2012. Why wasn't it the last straw back then?
Because -- at least for me -- it was the first straw (he may have done things before then, but I only joined GAF in 09). I can definitely see an argument for leaving then, but I also think one has to be careful with this sort of stand: if you refuse to work with anyone who has done something inarguably bad, then you would fairly quickly find yourself working alone. They have to do a sufficient number of things that make you say, "I simply cannot overlook this person's flaws any longer."
Please note also that I'm speaking for the other moderators, and cannot say this was their thinking. But it was certainly mine, before the decision was made for me (thus making any profession of purity moot).