As a result of the defeat of their garrison/assassination of their governor, the Philistines would have realized that the Israelites were planning to take up arms against them to expel them from their land. The fact that Saul was credited with the attack against the outpost/governor was because what Jonathan did for Saul, or what he did on Saul's orders, is considered to be an action of Saul himself.1
4 And all Israel heard it said that Saul had defeated the garrison of the Philistines, and also that Israel had become a stench to the Philistines. And the people were called out to join Saul at Gilgal.