There is nothing wrong or objectionable about what Mr Cameron stated; the Paksitanis delude themselves and try to pass the delusion to the rest of the world by stating that the 'problem' of terrorism started in the west.
The mujahideen definitely helped the West by fighting the Soviet invasion, but after the withdrawal of the Soviets, these same mujahideen, many of whom are now considered to be part of Al Qaeda and/or the Taliban were supported and nurtured by Pakistan and Saudi Arabia. It was Pakistani support for these mujahideen and Pakistani created madrassas that have caused most of the problems since 1991.
It's one of the worst kept secrets in the world that the ISI, along with the Pakistani military that provided the logistical, and material support along with monetary from Saudi Arabia for the Taliban as they attempted to expand their Wahabbi inspired rule over all of Afghanistan. It's no surprise that the only countries to give official recognition to the Taliban ruled Afghanistan was Pakistan, Saudi Arabia, and the UAE.
The Pakistanis are playing a double game, of course, everyone plays a double game, maybe the Pakistanis are just bad at it that they keep getting caught and can only offer pitiful excuses for their behaviour.