The cloud was just a field emitted by something. The Klingons realized this and reported back, while Epsilon IX watched, that they thought there was an object at the center of the cloud and that
was their target.
Saying it's ridiculous to even think you could attack the cloud because it's so big is a bit like saying it's ridiculous to attack the Earth with a hunting rifle. Well, yeah, but if you're actually planning to shoot a buck that happens to stand on the surface of the Earth, it starts to look a little more reasonable.
And I'm not criticizing as such, but I find captrek's
original definition of an A.U. being "about 100 solar diamters" a little odd (pedantically, it actually works out to ~107). Odd because the original definition of an A.U. was the distance from the Earth to the Sun (well, actually the length of the semi-major axis, but we're among friends and the like here). And that's usually the scale people work with ... 8 light-minutes, about 149 million kilometers, around 93 million miles, etc. when they define what an A.U. is. Why did you settle on using solar diameters, captrek