You can always go back. The problem is, as was alluded to above, your backups all have the same name where they are stored (on your PC/Mac or in iCloud). Unless you made a backup and archived it (i.e., gave it a different name) prior to moving to the beta, the first time your phone backed itself up on iOS 12, it would overwrite your most recent iOS 11 backup (due to file having the same name). Frankly, this is becoming less and less of a problem with the amount of data that is either backed up or replicated in the cloud. But you may have stuff you don't want to lose. So, if you are thinking of going back, I'd archive that iOS 11 backup before the phone can do an iOS 12 backup.
As for the method the video shows, you may not lose data, but if your backup has already become an iOS 12 backup, you won't be able to restore from it (until you are back on iOS 12). So you might lose any activity or health data that you accumulated while on iOS 12.