Good point, but I read that as being more that 400W were going into the neighbor's window or up into the sky, and the 100W is intended to keep same light levels

Good point, but I read that as being more that 400W were going into the neighbor's window or up into the sky, and the 100W is intended to keep same light levels as before. If you don't need those light levels, a 50W floodlight would cut the remaining light pollution from upward reflections in half.