Mr. K is saying that those who have complained about his roofing company are biased. The author's argument is claiming those who disagree with K could still be treated badly, so really it's Mr. K who is just mean. As a result, the author concludes that Mr. K's claim is completely false.

The conclusion is extreme compared to the support. Essentially, it's saying that Mr. K's argument is false because regardless of the political outlook of the complainants, Mr. K will still be a bully. But what if it's the complete opposite? Maybe Mr. K is actually nice. The argument is attempting to weaken Mr. K's claim but then treating it as if Mr. K's argument is completely wrecked.

Here's a different way to think of it: If you think of it on the spectrum from J.Y., B eliminates the possibility of the kid paintings to land on the second half of the spectrum, i.e. displeasing. So if most if most of kids paintings were NOT displeasing, then for any other painting that is "better" than a kids paintings, it makes it more likely that it is aesthetically pleasing. This helps close the discrepancy between being the MOST aesthetically pleasing, as oppose to just being MORE aesthetically pleasing than the kids paintings.