Neither the original F-15 nor the F-15D mentioned in the article have fly-by-wire, which is considered as a feature for the updated F-15SE Silent Eagle.
So interesting aerodynamics and mad flying skills, not fancy electronics :-)
The F-15 has a Control Augmentation System (CAS), and would be difficult to fly without one: http://www.f15sim.com/operation/f15_flight_control_system.ht.... Fly-by-wire means there is no hydraulic/mechanical control system, and is a relatively new feature. But computers have been "in the loop" with conventional hydraulic/mechanical control systems for quite a bit longer, especially in fighter planes, where the emphasis on maneuverability calls for airframes that are a lot less statically stable and would be hard to fly without control augmentation.
It has an adaptive electronic control augmentation system which while not a true fly by wire system still players a huge role in keeping the thing fly able.
When people use the phrase "fly by wire" they usually mean a system where the control stick feeds a digital signal into a computer, which controls the hydraulic servos for the control surfaces directly. The F-15 doesn't have that. Instead, what it has is a system where the stick is connected mechanically to the hydraulic system. On top of that, two electronic systems (the Pitch/Roll Channel Assembly and Aileron Rudder Interconnect) can provide an additional variable control input to the hydraulic actuators, to even out the feel of the stick in varying flight conditions. It also provides inputs to improve the dynamic flight characteristics of the plane: http://www.f15sim.com/operation/f15_flight_control_system.ht... (see the paragraphs under "Control Augmentation System.")
So the F-15's control system provides additional electronic input to the control services to improve flight characteristics, but isn't totally in charge of the airplane in the same way as a true fly by wire system. For example, the electronics cannot move the control surfaces through their full range of motion, and are strictly reactive to the pilot's stick input.
So interesting aerodynamics and mad flying skills, not fancy electronics :-)