Although it is true that Iraqi intelligence was quite effective in rolling up human intel networks, they didn't get all of them. The CIA had a station running spies out of Kurdistan in norther Iraq in the months leading up to the invasion. It had contact with Kurds who had infiltrated the al Qaeda fortress.

As for population density, it's not all that difficult to see the way that terrorists streamed into Iraq after the invasion. They trekked in from all nations of the region and even farther afield. Iraq became a hotbed of terror attacks.

The CIA's HUMINT was also good enough to infiltrate Saddam's security forces at Doha Farm in preparation for an assassination attempt at the start of the war. The inability of the CIA to collect intel in Iraq is greatly exaggerated.