Corbyn doesn't support appeasement.
Thank you for not going down the evil doers road, that would have done my head in at this time of the morning. I have no truck with the motivations of the bombers, but the brain washed notion is just a handy gimmick used by our political masters to avoid any analysis of why the bombers do what they do. There is a long list of grievances real and imagined in the Middle East, from the Balfour Declaration, the foundation of the State of Israel, the American engineered coup in Iran that ushered in the Shah, the Iranian Revolution and the vehement anti Americanism, the First Gulf war, the Iran Iraq war, the second Gulf War, the conflict between Sunni and Shia. Western support for various dictatorships in the Middle East and North Africa (particularly Saudi with their strict Wahhabism). I worked the Middle East for over twenty years and I travelled extensively there, to go through the various grievances and feuds would take pages.
The 7/7 bombers were mostly of Pakistani origin and as you say they were clearly not Arabs, they were united by their religion,which transcends their nationality, they see the West as an enemy of Islam, once you plug in to that as Bin Laden and Al Zawahiri did, then national boundaries between Muslim states are just lines on a map.
You clearly think that anyone who sees the West as an enemy of Islam is brainwashed, that's not how they see it, even if you forget the West's troops on the ground, the bombs, the oil we covet so much, there's our pervasive, liberal (permissive) cultural imperialism, poisoning the minds of the young with consumerism and other temptations, you might think this is all a bit middle ages, guess what, you're right, but it is no less real for that.
The principles of cause and effect apply in foreign policy as they do in all other walks of life, all Corbyn is saying is that the black and white argument that you either have the war on terror or appeasement is a false one and the path of dialogue and the search for a political solution is not only necessary, it is essential.