Ken Feldman from Santa Monica clarified about the 8 hours long gameplay. Here is the quote:
Reviews are awesome, we are humbled as developers. I think around 40% are perfect scores, unbelievable!!!
The one issue that has irked me is some reviewers are listing this as an 8hr game. This is not an 8 hr game, the fastest play tester on normal mode took 12 or 13 hours during the final play through - the fastest!
I have no idea why or what the motivation is to put out misinformation.