Honestly, if people beat it in 5 hours then they played it wrong. There isn't any joy in just cloaking and avoiding any gunfire through the whole campaign. Same could be said about just running in guns blazing like CoD. Any game can be short if you just sprint to the finish line.
I'm midway through the 4th level and at 7.5 hours in. I can see me finishing the game with at least 12 hours clocked in.