Hidden deep in the leaked Guantánamo files is a small but important trove of information, too historical and too technical to have commanded much space in newspapers keener on hyperventilating about "nuclear al-Qaida hellstorms" this week. Each of the 700-plus files includes a short biography of its subject. These cover his "prior history" and "recruitment and travel" to wherever he became fully engaged with violent extremism and, with brutal if unintended efficiency, demolish three of the most persistent myths about al-Qaida.

The first is that the organisation is composed of men the CIA trained to fight the Soviets in Afghanistan who then turned on their mentors. In fact among the bona fide al-Qaida operatives detained in Guatánamo Bay there are very few who are actually veterans of the fighting in the 1980s, and none of these were involved with groups that received any substantial technical or financial assistance from the US, even indirectly via Pakistan.

The second is that an "international brigade" of Islamist extremists was responsible for the Soviet defeat. The records make it clear that their combat contribution was negligible.

The third myth is that most of those currently waging "jihad" against the Crusader-Zionist alliance or the "hypocrite, apostate regimes" of the Muslim world were actively recruited by al-Qaida and brought, brainwashed, to Afghanistan to fight or be trained. The descriptions of almost all those in Guantánamo genuinely associated with al-Qaida shows that in fact they spent much time and money overcoming many difficulties to find a way to reach al-Qaida. They were not dumb or vulnerable youths "groomed" to be suicide bombers; they were highly motivated, often educated and intelligent, men.

Such details are easy to dismiss as irrelevant to the threat posed by Islamist militancy today. But they are not. For one of the elements marking the evolution of the discussion and analysis of the phenomenon that al-Qaida constitutes is the extraordinary degree to which it has been informed by myths.

There have been various waves of mythmaking about al-Qaida. The first wave came in the late 1990s, when the group gained international notoriety with attacks on US embassies in east Africa and a warship off the Yemen.

It was then that the idea that al-Qaida was "blowback" from the Afghan war became conventional wisdom. After 9/11 came a new, massive surge of fearful fantasy. There was the normal derogatory propaganda expected in wartime – that Bin Laden has deformed genitals or partied wildly with prostitutes – which gained no real purchase. A more pernicious myth was the idea that al-Qaida was a "tentacular organisation" with sleeper cells across the world waiting for the moment to strike with weapons of mass destruction. This minimised the role that both ideology and a variety of historical factors (ranging from demographics in the Islamic world to a discourse that stressed the "humiliation" of Muslims by the west) had played in the success of the group.

The emphasis on the agency of Bin Laden and his entourage discouraged interest in the broader causes of terrorism and thus made the fundamental strategic errors made by US and other policymakers in the early part of the last decade much more likely to happen.

Many myths were deliberately generated by governments. In 2002 and 2003, repressive and dictatorial regimes around the world scrabbled to uncover or rebrand local militant movements with long histories as al-Qaida offshoots. New Delhi claimed that Bin Laden, a 6ft 4in Arab and one of the most recognisable fugitives for centuries, had hidden in Kashmir, a smallish part of India crawling with 500,000 soldiers and police.

The Russians claimed the Chechen conflict was not about centuries of territorial wars in the Caucasus but about "global jihad". The discovery of a local branch of al-Qaida guaranteed major financial, diplomatic and military pay-offs from Washington – or at the very least a blind eye turned to domestic repression. So the Macedonians rounded up some Shia Pakistani immigrants, clothed them in combat outfits and shot the "al-Qaida operatives" dead.

Finally, there were the most egregious examples of mythmaking: the spurious connection of al-Qaida to Saddam Hussein and the non-existent weapons of mass destruction.

Most of the documents in the Guantánamo files date from 2003 to 2005, and reflect the concerns of the time. The assessments of each detainee reveal a particular focus on the threat of a mass casualty attack involving chemical and biological weapons.

For those who remember the headlines announcing al-Qaida plans to smear ricin, a poison, along parts of the London underground nine years ago, the stories this week announcing the existence of an al-Qaida nuclear device hidden in Europe, or that London is a "hub" for al-Qaida activity, seem from a distant era when every reported scare provoked panic. Indeed, many of this week's scare stories, based on the "confessions" of tortured Khaled Sheikh Mohammed, were leaked at the time, and now appear to have been made up or even the product of al-Qaida's own myth machine. Now, thankfully, the public is better informed and less prone to being scared. Given that terrorists' primary aim is to terrorise, this is to be welcomed.

The events of this spring have shown that Bin Laden and his cronies are definitively drifting to the geographic, political, cultural and ideological margins of the Islamic world. Their attempt to radicalise and mobilise hundreds of millions of people has failed. Crowds shouting slogans of democracy, not of violence, have succeeded in forcing the departure of two dictators and shaken several more. The Arab spring started with a public self-immolation, an act of spectacular violence which impressed because it harmed no other and was thus a clear repudiation of the suicide attacks of the last decade. The few statements from al-Qaida's leadership or affiliate groups have sounded tired and irrelevant.

One reason for the group's current weakness is the gradual unpicking of the myths that contributed to the fear al-Qaida once inspired and the aura it had for the alienated and the angry of the Islamic world. Happily, it is unlikely those myths can be rebuilt.