Had reigned - Rather, “had ruled.” It is not the phrase used in Judges 9:6. It looks as if the Shechemites alone had made him king, and the rest of Israel had submitted to his dominion, without allowing his title of king.
Matthew Henry
Concise Bible Commentary
Abimelech is seated in the throne his father refused. But how long does this glory last? Stay but three years, and see the bramble withered and burned. The prosperity of the wicked is short and fickle. The Shechemites are plagued by no other hand than Abimelech's. They raised him unjustly to the throne; they first feel the weight of his sceptre.
SDA Bible Commentary (limited)
Volume 1-7
22. Over Israel. That is, over all the Israelites, who would accept his authority. This comprised probably chiefly those in the area of Shechem.