Geek.com was the source of the rumor pertaining to Kingdom Hearts III's playthrough length. They had hands-on time with a demo, and used that as a basis to make a hypothesis that the game would take 40-50 hours to complete and have a final tally of 80+ hours if gamers completed all of the subquests and minigames.
That sounded exactly like what gamers wanted to hear, which likely factored into why the rumor gained traction. IGN has recently reached out to Square Enix, who have confirmed that they've yet to actually finalize's the game's length. Since then, Geek.com has updated their article to reveal that they were only guessing.
It's July and Kingdom Hearts III doesn't actually hit retailer shelves until January 25, 2019- that's 6-months out. it's way too early to be making any definitive statements about the game's length.