This information comes from multiple sources, such as Kotaku and Gamnesia, both of whom have reportedly heard from their sources at GameStop that the game has been delayed. Apparently Square Enix has briefed GameStop’s executives that the game will be pushed to a November release, or to be more specific, the 29th of November.
That being said, Kotaku notes that apparently GameStop employees will go ahead and put up the standees and marketing materials that list the 30th of September date, but will later be instructed to cover up the date with a sticker that reads the 29th of November. It is unclear as to why Final Fantasy XV might have been delayed.
However delays for video game releases aren’t exactly new or uncommon, and usually this has to do with the game requiring additional polish, which can only be a good thing for gamers since no one likes to play a buggy or incomplete game, but until we hear from Square Enix officially, take it with a grain of salt for now.