I believe there is an apostrophe missing somewhere in his statement. While it might seem implausible to the Sony fans on these forums. There could be a valid argument that the multi player aspect might be exclusive to the 360 version. Time to dust off the tinfoil hat.
There are a myriad of reasons why the 360 might get the definitive version of the game. Perhaps the delay due to PS3 development gave the 360 team more time to add in additional features. While the PS3 version might be more visually stunning. The 360 version might support more features.
Which brings a question to mind what if developing the title for the PS3 has had an unintended side effect. The developer might have ratcheted up the graphic prowess on the PS3 version, but the price might be that the game for the PS3 could never be multiplayer without serious quality issues. Thats ignoring the architecture differences of the machines. While the PS3 is hardly an online machine. The 360 was built explicitly around the premise.
Another consideration is the exclusive 360 content for download. Had we learned anything from this past year about live its this. The best downloadable content is the one that accentuates a multiplayer experience. This perhaps was the intent of the downloaded content in the first place, and Microsoft having secured it for a hefty price might be all the explanation we need. Perhaps what Microsoft really bought was multiplayer for their GTA title.
Honestly there are many good reasons it might happen not least of which being that the 360 is the dedicated online machine. Which would make the greatest use of such feature.