Looks to me like a lot of them are slowing down to see what kind of performance art he’s doing. Even the bus driver pointed at him in curiosity. Admittedly, there are also a lot of them swerving away from the puddle in a moment of sudden awareness, or just swerving away from him in a moment of fear.
There may be some aspect of attention-grabbing that would work no matter what was in his hand. People see so many umbrellas on a rainy day that it barely registers, but if you hold anything at all that’s eye-catching, you’ll pop up in their conscious attention and they’ll treat you like a person instead of a road hazard.
I mean, if it works it works, but I think it would be less effective if a lot of people did it. “Oh, I’ve seen this one, he never actually throws the brick.”