How does the ballast affect the tire rating at all? If you overload the tire with ballast by hooking up a backhoe, you'll overload the same tire without ballast, probably worse.
The tire rating will be usually related to air pressure rating, not weight, generally speaking. Any weight rating is if the tire is at rated air pressure. If there is no ballast, then the amount of air space in the tire is far greater, which will allow more tire distortion to compress the air to a much higher level inside the tire, and assume more of the load on the destorted part of the tire. Liquid doesn't compress, meaning the amount of volume of air at a given pressure is far more effective to keep the tire the proper shape. This means the tire is going to hold its shape a lot better for the same amount of added weight on a loaded tire. There's a pretty good writeup about ballast and why it's recommended in the OM about that, actually, which sort of means the manual is contradicting itself. Why am I not surprised? If you look at the photos showing you which filter is what, you'll find they're completely backward (I found out the hard way). If they could get something that wrong, why wouldn't the folks writing the instruction book and translating the engineers' specifications get something else wrong?