Is adb rooted by default?
Not at all. It just depends on which files you are working on.

Protected system files will need elevated privileges.

For example a simple installation via ADB does not require root eg

adb install com.package.name


While advanced commands like mounting in system dir needs root privileges.

e.g mount -o bind /mnt/emmc /sdcard/external_sd

Requires su permissions

As for your operation, interfering with system files needs root