«الْفِعْلُ مَعَ الْمَفْعُولِ كَالْفِعْلِ مَعَ الْفَاعِلِ»
The verb's relation to its object resembles its relation to its agent.
This is not a claim that subject and object have the same grammatical status. Rather, an action may be considered from the side of who performs it and from the side of what it affects.
ضَرَبَ زَيْدٌ عَمْرًا
Zayd struck ʿAmr.
The striking originates from Zayd and affects ʿAmr. The conversational purpose may be to describe both relations, or only to establish the action as proceeding from Zayd.