Doesn't sound like it's a 5 hour game.
Can't validate a personal opinion since I'm currently waiting on my copy, but I have played the previous 2 games quite extensively and I imagine my play habits on how I approach the game will be similar, if not the same.
I didn't bother counting the hours it took me to finish either of the previous games. Needless to say, neither of them were too short or too long and I certainly didn't finish them in a single extended 5 hour session.
If I were to base my expectations on the claim of the OP, this is what I will expect; that I can finish the game in 5 hours. And no, I'm not going to try and race through the game for the same reason that I don't watch movies in fast forward or to see how fast I can eat a sashimi dinner. That's not what I'm paying for.
MGS5 is a prologue. Not a demo. Yes, it's short if you just do a quick play through, but it was designed with plenty of little toys to play with in game, much like MGS4 to extend the value. Plus, it's a primer for the main game, meaning it's a prequel stage that won't be a part of Phantom Pain.
Kind of a cheap move by Konami, but it serves the dual purpose of promoting the Phantom Pain since the game is still quite a ways off, and it gives fans (and this is really for fans of MGS who don't want to wait) something to play with in anticipation of MGS5 proper.
But no, it doesn't compare to a full retail game like Infamous SS, even if the average player can finish the game in 5 hours on regular difficulty, which I am unconvinced is the case.