If this is not required by systemd, why is it done by sd_notify ?
It's not.
You obviously did not read the code. It is. Here is a G+ convo with Lennart I had:
> As a sender you only have to set SCM_CREDENTIALS manually if you want to
> fake it (for which you need privs however).
sd_notify() basically impersonates the process. You only need to set SCM manually if you are writing an external library. If someone is just doing it in the daemon, the kernel will set SCM_CREDENTIALS automatically.