U.S. Military: Iran Working With Sunni and Shia Insurgents

According to VOA:

The U.S. military has presented evidence that it says shows Iranian intelligence agents are providing weapons and training to both Sunni and Shi'ite insurgents in Iraq.

But I thought Sunni and Shia couldn't possibly conspire to kill Americans due to "irreconcilable theological differences?" Steve Shippert deconstructs that argument here, and points to this letter from al Qaeda #2 Ayman al Zawhiri's to the late Abu Musab al Zarqawi:

And do the brothers forget that we have more than one hundred prisoners-many of whom are from the leadership who are wanted in their countries-in the custody of the Iranians? And even if we attack the Shia out of necessity, then why do you announce this matter and make it public, which compels the Iranians to take counter measures? And do the brothers forget that both we and the Iranians need to refrain from harming each other at this time in which the Americans are targeting us?

The Iranians and their Hezbollah proxies may one day go to war with al Qaeda over theological differences, but for now, they seem quite capable of putting those differences aside in favor of killing Americans in Iraq.