Twrp backup is not the same as a flashable zip. To RESTORE a backup, you must use the RESTORE function. If you are attempting to restore a backup from another phone, then you first need to make a small backup with the phone that you wish to add the backup to. This will create the proper folder name within the twrp backup folder. So it will look like: TWRP/BACKUPS/ lgl34cabc123xyz, or something similar. The last folder name has a different code on the end for each individual phone, so this is necessary. Then place your backup inside of that folder, reboot to recovery, choose restore, select your backup, and restore it.
It would be nice if TWRP didn't work like that or at least gave the option to disable it. Would've saved me a lot of time..