I believe him. You can't stop what you haven't started, and even when Fatah was in charge the Palestinian Authority showed little enthusiasm for cracking down on terrorism. Now that the terrorists themselves are running the show, the Palestinian leadership is experiencing what The New York Times calls "tension:" Abbas wants to pursue terrorists, and Hamas wants to pursue terrorism.
Since words are all Abbas has to offer, it would have been nice to hear something about how murdering people at random is, you know, wrong, not just imprudent. But since the Fatah-affiliated Al-Aqsa Martyrs Brigade not only praised the bombing but (according to Reuters) tried to take credit for it, I guess Abbas is going out on a limb even by suggesting it was not a smart move.
In fact, with Fatah's terrorists and Islamic Jihad (the group that actually sent Hamad to Tel Aviv) continuing their attacks, perhaps it's Hamas, which has not carried out any in the last year, that counts as moderate these days. Many Fatah members think their route back to power is paved with body parts.
Although this situation seems untenable, it can continue indefinitely. The terrorists can cause Israel pain, but (unlike, say, a nuclear-armed Iran) they do not pose an existential threat. The Israeli government can continue its program of unilateral separation, which has wide popular support, and maybe someday Hamas, like Fatah before it, will pronounce itself ready for negotiations. Given how that scenario worked out the first time around, maybe it doesn't really matter.