This video doesn't prove your point. Not only are they actually not directly under that, but this is spread over a much larger distance than trying to target the cathedral would be. They configure their release to spread over a larger area than that.
It proves you can effectively firefight with water from a plane without even snapping tree branches, thus there'd be no reason the same pressure couldn't be used on a building.
That's much more volume at once in any one area and much closer to the subject (so less time to dissipate) than any of the actual flying water tanker videos that have been posted.