Have you read the statute in question?
50 USC 1801 (definitions)That's it - that's all of the material in 1801(a)(1) - (3). And without more, perhaps an argument can be made that terrorists, or more narrowly, "international terrorists," fit those definition. But the statute itself goes on to add the following definitions. Note that Section 1802, warrantless surveillance, applies only to the above referenced, and does not specifically recite that warrantless surveillance can be conducted against 1801(a)(4) targets.(a) "Foreign power" means--
(1) a foreign government or any component thereof, whether or not recognized by the United States;
(2) a faction of a foreign nation or nations, not substantially composed of United States persons;
(3) an entity that is openly acknowledged by a foreign government or governments to be directed and controlled by such foreign government or governments; ...
50 USC 1801 (definitions)Now, one might ask why Congress decided that surveillance against foreign terrorists requires a warrant. That is, why did Congress draw the line where it did? But that's a Congressional call.(a) "Foreign power" [also] means-- ...
(4) a group engaged in international terrorism or activities in preparation therefor; (5) a foreign-based political organization, not substantially composed of United States persons; or (6) an entity that is directed and controlled by a foreign government or governments.
At any rate, Congress went a little bit further, and defined international terrorism as well.
50 USC 1801 (definitions)How can it be that it is a violation of FISA to conduct warrantless electronic surveillance on terrorists?(c) "International terrorism" means activities that--
(1) involve violent acts or acts dangerous to human life that are a violation of the criminal laws of the United States or of any State, or that would be a criminal violation if committed within the jurisdiction of the United States or any State;
(2) appear to be intended--
(A) to intimidate or coerce a civilian population;
(B) to influence the policy of a government by intimidation or coercion; or
(C) to affect the conduct of a government by assassination or kidnapping; and
(3) occur totally outside the United States, or transcend national boundaries in terms of the means by which they are accomplished, the persons they appear intended to coerce or intimidate, or the locale in which their perpetrators operate or seek asylum.
If the terrorist act are being planned or undertaken by an entity within the definition of "foreign power" as recited in 1801(a)(1) - (3), then surveillance without a warrant is within FISA boundaries.
The analysis of 1801(a)(1) - (3) then moves to a definition of "foreign government or component thereof" (Taliban satisfies this), "a faction of a foreign nation," and/or "an entity openly acknowledged, directed and controlled by a foreign government." Examples would be useful, but unfortunately I have no certain ones. I speculate that perhaps Hamas is so classified, and that Al Qaeda is not so classified. There are also a good number of other terrorist organizations that no foreign government openly acknowledges, directs and controls.
The article that you referenced in your post spends quite a bit of time getting an actor to come within the scope of "agent of a foreign power." That is separately defined under 50 USC 1801. Being so classified would -NOT- support a warrantless electronic surveillance under the terms of 1802. But attaching the label "agent of a foreign power" to an actor does not result in justifying warrantless surveillance under FISA.