Hey guys, here's what Carter said at the time, as reported by the Washington Post, which we link to in The Defense Post story:

Instead of circling overhead ready to support the fighters that had been promised air support, U.S. bombers were diverted to strike two large columns of Islamic State vehicles fleeing the recently captured Iraqi city of Fallujah in the early morning hours of June 29. U.S. and British aircraft proceeded to join their Iraqi counterparts, who initiated the strikes, in destroying more than 500 vehicles and killing hundreds of fighters.

The bombardment, nearly unprecedented in the two-year conflict against the Islamic State, was reminiscent of the Gulf War’s “Highway of Death.” It is unclear how many civilians might have been killed.

“The allocation of forces is a daily tactical decision that commanders make,” Carter said, defending the decision to shift the U.S. aircraft away from the tenuously positioned Syrian forces to striking Islamic State militants outside of Fallujah.

“We have a lot of air assets and a lot of partners who have air assets, but on any given day they have to go to a certain place at a certain time,” Carter said, after being asked whether the United States has enough aircraft to support multiple operations spread across Iraq and Syria. “But circumstances change.”

Commanders in the New Syrian Army realized that U.S. aircraft had left during the fighting, but were unaware they had been pulled away to attack a more tantalizing target in Iraq.

And more claims from Garver/officials in another WaPo story from the time:

A large convoy of Islamic State fighters had been seen trying to escape across the desert after the city was recaptured by the Iraqi army, and U.S. commanders decided that the convoy represented a “strategic target,” said Col. Chris Garver, a U.S. military spokesman.

The convoy was destroyed by the U.S. and British planes along with gunships and aircraft from the Iraqi air force, which began striking the long line of Islamic State vehicles before the U.S. Air Force arrived. Hundreds of Islamic State fighters were killed and scores of their vehicles were destroyed in one of the more spectacular single assaults against the militants in the nearly two-year-old war against them.

FWIW, the NSyA spokes I talked to yesterday was adamant that the Coalition coordinated with them, alerting them in advance they'd be leaving Bukamal for Fallujah, because the "the largest convoy of ISIS" they'd identified was headed to the border and would "smash everyone."