It means something else - it's a comparison. So in your example "he hates kids even more than he hates dogs, which is saying something" the person saying that sentence knows that the man really hates dogs, so for him to hate kids more than that is quite significant. To give another example, if I said about myself "I love bacon even more than I love chicken, and that's saying something." I'm telling you that I like chicken a lot, but that I like bacon even more than that, so my love for bacon is very significant. You generally use it with a friend, or someone who is familiar with the subject you're talking about. In my example I wouldn't say that to someone I didn't know; I'd say it someone who already knew how much I liked chicken, so that when I tell them I like bacon even more than that they would understand the significance of it.