I think his point with the Humvees was that we left those in the care of the Iraqi military. I've served in Iraq, everyone there knew for god damn sure that giving those guys guns was a stupid way to get out of Iraq, but it was the best shot we had at not making it the 51st state because of the poor planning on W's part. Since Paul is very much so against expeditionary warfare, his argument is more along the lines that we shouldn't have been in a foreign country for no reason in the first place which forced us to leave thousands of military vehicles (entire fully built-up bases were left in place) in the hands of an irresponsible military. Expeditionary warfare leaves all of these possibilities wide open, and in a not so indirect way should be understood as helping future enemies by giving extravagant weaponry to people who can't (or won't in the case of the Iraqis) defend it.
I just think he framed it disingenuously, because he made it seem like we were supplying ISIS allies with humvees and that's how they acquired them. Even if I agree that we arm groups in an irresponsible fashion, ISIS didn't acquire its weaponry through trade with other groups.