This website, https://circuitglobe.com/overhead-ground-wire-or-earth-wire.html, said for effective shielding, we should keep the protecting angle as small as possible, the angle between 20° and 30° is quite safe, and it should not be kept above 40°.
Why? I think the larger the protecting angle becomes, the longer will the \$L\$ be, that is, the range that shield wire can protect will become larger. Isn't it good to let \$L\$ become longer? Why does the website say we should keep the protecting angle as small as possible?
The article you've linked to states:
Figure 1. The rolling ball lightning protection model rolled against the protecting angle model. I have assumed that the tower is 20 m tall. (Combined image mine.)
The lightning protection angle is a simplification of the lightning rolling sphere method of analysis. To get 40° lightning protection angle may require you to assume a large sphere diameter which will reduce the safety factor of your analysis. 60 m seems to be a common value used and if my assumption of 20 m for the pylon is close then you can see that the pylon design is almost a perfect fit for the 60 m sphere.
I know only a little about the subject so wait a few days before accepting this answer if you are so inclined. That way you will encourage others to answer and may get some other insights.