Square Enix has shared their plans of the post-launch support for the game, and it includes new additions to the story cutscenes so that the character motivations are explained in detail. While we haven’t received any major update related to it, it appears that one of the recent update is slowly implementing some changes in the open world portion of the second continent, that is only explored through a glitch.
This could indicate that the content was NOT unused content from the main game, but rather areas that will be used in the upcoming DLC and content coming to FFXV in the near future. So people able to experience the content through the reported glitch are just getting early access to some of the new areas they can explore with Noctis’ party members, as they will be the focus for the upcoming character-focused DLC.
Source: http://gearnuke.com/final-fantasy-xv-patch-1-03-reportedly-adds-roads-open-world-exploration-niflheim/