Here

https://www.zabbix.com/documentation/current/manual/config/items/itemtypes/zabbix_agent

on this key

system.hostname[<type>, <transform>] 

Description

The value is acquired by either GetComputerName() (for netbios) or gethostname() (for host) functions on Windows and by "hostname" command on other systems.

on this part

and by "hostname" command on other systems.

Are not correct

Formally - agent in src/libs/zbxsysinfo/+/hostname.c SYSTEM_HOSTNAME() function calls system API uname(). From all names returned by uname() Zabbix agent takes nodename and returns it as hostname.