Translation:This is not the eastern, but the southern hemisphere.
I didn't know there were more types of hemispheres than just northern and southern! Now I feel stupid, haha. Or was that an error?
Sure, you can split a sphere in quarters! Both sides of the equator as well as both sides of the prime meridian, which runs through Greenwich, UK. This sentence doesn't make a lot of sense though, since you can be in the southern and eastern hemispheres at the same time (see: Australia)
This sounds strange to me. It seems like it's saying that you wouldn't expect for something out of the eastern hemisphere to be in the southern hemisphere. If someone had just said that it was in both the eastern and the southern hemisphere, and you wanted to correct the one and confirm the other, I would use "This is not the eastern hemisphere, but it is the southern one."