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.