The Tuesday tweet stated that the earlier report of a Galaxy S IV announcement at the 2013 MWC in February was untrue, as was information about a global launch in March. The tweet then asked why anyone would want to spread such a rumor.
That is a good question, and the answer is unknown. However, if you instead asked why Samsung responded so quickly, the answer is pretty obvious. The Samsung Galaxy S III is the company's top of the line device, and despite what Apple fanboys might think, is a worthy opponent of the iPhone 5.
That said, Samsung would naturally be concerned that rumors of an impending successor to the GS3 would slow sales. That's natural, and had already been seen with sales of the iPhone 4S, which slowed as the rumored launch event of the next-generation iPhone approached.
The source of the original report, the Korea Times, said it had received its information from Samsung officials, which adds a "huh?" as a denouement to the story.
There's no doubt that there will be a Galaxy S IV released next year. The question is not if, but when. While an earlier release of the device will surely blunt iPhone 5 sales more than a later one, if information about a GS4 introduction gets out too soon, it may also blunt sales of the GS3.
So it may be that both the original rumors, and the denial, came from the same place: Samsung.