1. Funding the opposition, is not the same as fund a coup or arming factions in a civil war. The US funds opposition movements in many countries. The link you give is about the US performing a raid into Syria against an Iraqi insurgent network.
2. What happened in Syria is a civil war not a Coup d'etat.
>"the Syrians who had been trained, armed, and radicalized by the CIA fled to Iraq when the coup failed and started ISIS."
The civil war was going well for the rebels all throughout the rise of ISIS, so I'm curious when you thought this was happening? All sources agree that ISIS was formed in Iraq and then infiltrated into Syria.
"the US and its allies weren’t only supporting and arming an opposition they knew to be dominated by extreme sectarian groups - they were prepared to countenance the creation of some sort of “Islamic state”"
Lets parse that carefully as it is deceptively written:
* "the US and its allies" that is, the US had one policy and its allies had another, but the author chooses to group them together to blame the US for activities undertaken by US allies which the US actively opposed and ran counter to US interests.
* "supporting and arming an opposition they knew to be dominated by extreme sectarian groups" while true the US was going out of its way to arm the less extreme elements (unlike some US allies).
* "they were prepared to countenance the creation of some sort of “Islamic state”", that is, a DIA intelligence assessment considered the possibility that the Sunni population of Iraq might break away from Iraq. They didn't support this possibility, just said it could happen (and it did). This state need not be "The Islamic State", but author choose the name "Islamic State" to make it sound like the US supported the Islamic State.
The author of that piece is being willfully misleading. Note that it was published in the Opinion section of the guardian. Even with all that the author supports my claim that ISIS started in Iraq.