Runner behind him isn’t out yet. Once that runner is out, or the guy touches the bag or tags the batter, then your guy standing on first is safe. Prior to that he doesn’t have the protection of standing on the wrong base from being tagged out. The link shared above explains this a bit when it happened and they touched the base before tagging the runner, had they tagged the runner standing on first and then touched the base it would have been a double play. The first baseman was confused