He addresses it to an extent early on:

The current rise of ISIS has been made possible by the exclusionary practices and increasingly authoritarian tendencies of the Maliki government in Baghdad. Maliki's regime is a narrowly based Shiite regime, and most Sunni Arabs do not see a future for themselves in an Iraq led by Maliki. In such a situation, the extremist message has appeal. The political situation in Iraq is in this respect an unsurprising consequence of the political culture that was left when the Baathist regime was deposed.

Basically: Pre-invasion, Hussein's regime was comprised almost wholly of Sunni Baathists at the exclusion of the regional Shiite majority, who were viciously suppressed. In come the Americans, smash the Hussein machine into little tiny bits. Say to Iraq: "you're free. Now do the whole democracy thing." Only Iraq has only ever really been a loose confederation of fractious ethnic subgroups, held together by the glue of total oppression. The Sunnis don't trust the Shiites for associations real and imagined to their Iranian counterparts, Shiites don't trust the Sunnis because, you know, the last forty-ish years of rule, nobody likes the Kurds, who to the best of my ken are the red-headed stepchild of the region, and who've spent the last however long fighting for an autonomous state. Anyhow. Amidst all this, Iraq is supposed to pick themselves up after decades of autocracy, re-form a working bureaucracy without input from the deposed Baathists, and make a stab at any sort of representative democracy while ignoring the reality of a highly disproportionate distribution of sectarian groups who are very much at odds with each other.

Then Maliki climbs to the top of the power vacuum (our involvement in this is disputed... by Maliki), establishes a government that swings wildly into the direction of Shiite empowerment at the expense of the Sunni minority (a minority highly unused to not enjoying representatives with their hands on the reigns of government), and cracks down with increasing brazenness and violence on any voices of dissent, peaceful or otherwise.

Given that the state's monopoly on power is still tenuous, and that weapons are all but falling out of the sky, and that the whole of Iraq has been battle-hardened after eight years of chaos, and that the region has become both a beacon and a breeding ground for extremist militants... how was ISIS NOT going to happen?

So in this way, yes, ISIS is a direct result of the U.S. launching the Iraq war. No war, no immediate disintegration of Sunni influence. No disintegration of Sunni influence, no Sunni discontent. No anything else that happened in the last decade- normalization of violence, influx of instruments of warfare, power-grab from Shiite warlord bent on suppressing the Sunni minority, political situation that restricts all options for either power-sharing or legitimate dissent- and that Sunni discontent doesn't ever blossom into asymmetrical guerrilla warfare fueled by sectarian rage.

Also- don't think he was saying that there was decreased violence at the start of the occupation. Think he was saying that there was decreased violence around the time of the troop surge, i.e. towards the end of our occupation.