Author here. Yeah that graphic is a little confusing.
Since we have a 3x3 area of squares that are already accounted for by the current hit, we only focus on expanding that 3x3 area into a 3x4 or 4x3 area by shooting adjacent squares.
We're expanding the 3x3 to 4x3, so we only focus on the delta between the 3x3 and 4x3 area (hence highlighting the edges of the 3x3 area). Once we decide which direction would result in the most potential ships being eliminated, we shoot in that direction.
Since we have a 3x3 area of squares that are already accounted for by the current hit, we only focus on expanding that 3x3 area into a 3x4 or 4x3 area by shooting adjacent squares. We're expanding the 3x3 to 4x3, so we only focus on the delta between the 3x3 and 4x3 area (hence highlighting the edges of the 3x3 area). Once we decide which direction would result in the most potential ships being eliminated, we shoot in that direction.
Hope this helps!