I don't think that this was the case because the letters the hijackers had reminded them to bring their knives.
With increased scrutiny at security checkpoints, assistance from ground crews becomes more likely though.
And then there are all of the businesses that operate between the metal detectors and the boarding gate. Most probably have some cutting implements (restaurants, newsstands, gift shops) for use by the employees if not the customers.
If you are really determined, an aluminum can (from the beverage cart on the plane) could be torn into a sharp edge.
But boxcutters were found on at least 3 grounded planes after the fact on September 11. Either potential hijackers quickly hid them in case they were searched when they returned to the gate after the flights were canceled, or someone hid them before the passengers boarded the plane for in-flight use later. 12 flights that returned to the gate September 12 had Arabs sitting in the jump seat in the cockpit as "courtesy guests."