I found more info here: FreePBX 17 using Debian beta ISO - OSO missing Sysadmin
It seems it indeed is free but we cannot get it because it is commercial module. I guess it is available only with commercial FreePBX. Documentation says nothing about all of this, it actually says that System Admin is standard module and that here is PRO version that is commercial.
Even more, to install System Admin, Firewall module has to be already installed. Firewall module cannot be installed if System Admin is not already installed. Dead loop.