I don't know if it's disingenuous, but the problem for Corbyn there I think is the phrase "by whatever means they find necessary." By October 2004 it was pretty clear that the nature of the Iraqi 'resistance' was being shaped by the likes of Zarqawi. It's similar to John Pilger writing around the same time that "We cannot afford to be choosy. While we abhor and condemn the continuing loss of innocent life in Iraq, we have no choice now but to support the resistance..."
By all means, it was absolutely correct to criticize the disastrous war and call for the troops to be brought home. Many analysts and even ordinary people could tell before March 2003 that the war was being promoted along dishonest lines and would be a disaster for all concerned. In the same way, however, by late 2004 it was equally clear that the Iraqi 'resistance' was dominated by Al-Qaeda types. Now while that was obviously a function of the invasion and the blame lay primarily with the coalition, for some reason Corbyn and Pilger had to go that step further and proclaim support for a movement which was then getting into the business of beheading aid workers, blowing up the Shi'a, and attacking the UN.