Al-Qaeda and Osama bin Laden had long been carrying out attacks against American interests, and had made threats of future attacks. A well-informed news producer, journalist or member of the public could reasonably make the educated guess that the al-Qaeda was responsible, because it fit with their past actions.

To put it into context you have to understand that al-Qaeda had carried out multiple attacks against American interests in the 21 months prior to 9/11. On October 10, 2000 the USS Cole was refueling in Aden Harbor, a pair of suicide bombers approached in a small vessel and were able to detonate their explosives hulling the ship; killing the bombers and 17 sailors.

Ten months prior to this attack al-Qaeda had orchestrated multiple attacks around the globe to mark the 2000 Millenium: Including a failed attempt to bomb the USS The Sullivans, an attempt to bomb LAX (foiled on December 14, 1999), a failed attempt to bomb multiple targets in Jordan, an uprising in Syrian controlled Lebanon and a successful hijacking of an Indian Airlines flight.

In 1998 al-Qaeda affiliates had bombed two American embassies, one in Nairobi and one in Dar es Salaam.

So for at least three years prior to 9/11 it was publicly known that al-Qaeda was targeting American and allied interests and that the attacks were growing more brazen.

I think that you are also giving more credibility to the caller than they deserved regarding the association with Palestinian organizations. That is likely a bit of free association, aircraft hijacking and broader terrorism in the American mind was intrinsically linked to Palestine before 9/11. It was a well known tactic of PLO associates, take over a plane, force it to land and negotiate to get people released. The caller likely just lumped them in together, not realizing that bin Laden was associated with a separate group.