I think we will have to wait to find out the truth unless you have some evidence one way or the other.
Can you confirm this is fake or real from a reliable source or are just speaking from what you feel?
Split screen in GTA4? Only on PS3? Would be the biggest kept secret in the console wars yet, the trolling that would happend after this on sites like GameFAQs and Neogaf would be immense.
This is a fake.
Why on earth would the X360 version not be able to run split screen but the PS3 would?
The only way that would be able to run on the PS3 and not on the X360 (because Xenos would certainly not be the bottleneck here), would be if they managed render most of the graphics they normally render on the RSX on the Cell. And the chances of R* having the developer talent to pull something like that is slim to none.