Simply put, the stall strips induce unsteady flow (usually vortex shedding of some description) in to the airflow over the wing, which can be felt as buffet, when the wing is a few degrees of AOA below the stall AOA. The resulting buffet will not cause a stall because it does not lead to dramatic and sudden separation of flow across the majority of the top surface of the wing. However, it does provide a warning to the pilot that he can feel through the controls and his seat, in time for him to avoid the stall. The Hawk has similar devices but still gives very little warning of stall in the landing configuration, which sadly has been a contributory factor in the loss of some of them.
Stall, on the other hand, is marked by the separation point moving forward rapidly towards the leading edge of the wing at the stall AOA, leading to separated flow over most of the upper surface of the wing. Buffet and stall are not the same thing.
Given the learjet's rather unpleasant stall characteristics, one can see why they opted for stall strips! Sadly, such is life for swept wing aircraft. Witness also the wing fences and the boundary layer energisers (vortex generators) in front of the ailerons, and one can see that spanwise flow and tip-stall was a problem for these aircraft.