Well, if you have an unlocked bootloader, you can use a custom recovery, then you can flash a SuperSU zip, you do not really need it. This is for the devices that are un-rootable right now. I do not have any Android device that cannot be rooted, so I cannot try (I still want to have my G4 as stock as it is now, bootloader is still locked). If I understood correctly, that method modifies the boot image (which to me is like systemless root) by using DirtyCow. It tricks the system to modify the boot image and the recovery. It should be a lot easier for older devices, even with a locked bootloader.