It's was a talibunny op. The N.Koreans did the same thing during the Korean War. It was planned and staged by the talibunny command for it's propaganda value. Whether they succeeded or not in the escape, they get the propaganda they're after. The outcome realized lets the leftists run with the propaganda ball too.
The "surrendering" Talibs from Kunduz knew the old fort very well, and knew that if it could be taken from inside, they would be able to stage a long and bloody siege, an "Alamo", on their way to meeting allah.
They got their wish.