TWRP now has 3(!) different competing implementations of SAR currently in use by maintainers:
1) /system as mountpoint in ramdisk/fstab and ANDROID_ROOT=/system, and possible symlink from /system_root to /system
2) /system_root as mountpoint in ramdisk/fstab, ANDROID_ROOT=/system_root, and possible symlink from /system to /system_root/system
3) /system_root as mountpoint in ramdisk/fstab, ANDROID_ROOT=/system, and possible symlink from /system to /system_root/system
Attempt to continue to accommodate this bullshit.