Nobody knows exactly what the blade hit on its way out, and what else in the cowling was cut off in what direction. It is entirely possible that a piece of something from/in the cowl ricocheted and hit the window. All it had to do was weaken the window enough that the differential pressure could do the rest. Since there was no debris visible inside the cabin, I consider this the more likely scenario.
Also, even a "tight" seat belt is NOT "TIGHT". If you've never strapped into an airplane for planned negative-G acrobatics, you don't know how loose even a normal "tight" seat belt is. Virtually any passenger will have the belt VERY loose in cruise, allowing for significant upward movement if enough force is applied...