First, I have to correct myself regarding the ball path. I fixed the view to the cam looking back (2?) all the time, disregarding the close frontal view (3?) which shows that the ball runs through the hazard below the bridge.
Thus, the bridge's surface is not involved, the ball "falls through the planks" at the near side, either surfing the water there or rolling smoothly through the alleged rough. Add a short run back and forth at or on the far slope - very strange ;)
On the rules side, it's a water hazard if so defined or if usually filled with water, see the definition (scroll down). Usually there would be yellow stakes or lines indicating the borders. These should include the slopes of the brinks, i.e. the whole depression. From shots failing the bridge we know that it IS a water hazard. The bridge is part of it as well as dry parts of the ground.
It's perfectly legal to play the ball as it lies (if you take care not to bang your head against the bridge) - cf. Bill Hass in 2011. More vids in youtube. Rule 26 defines the available relief options for a player (add one penalty stroke). Rule 24 is about obstructions, stating that the free(!) relief is not available in water.
I respectfully disagree to your conclusion though. WGT should simply treat the situation as a ball in the water hazard like they would do 5 yds left or right. They increase the intricacy by looking at the bridge, ending up giving a wrong ruling. Lucky you, this protects you against a possible DQ ;)