Surfer Girl and Skater Boy just cull their rumours from other sources.
The Timesplitters 4 one, for example, is derived from a comment David Doak gave in a GameInformer interview:
"GI: Are you thinking about putting TimeSplitters on the Wii, as well? What do you think about making a first-person shooter for the Wii?
Doak: I think it’s a good thing to do, and I think we want to put it on the Wii."
The "I think we want to put it on the Wii" part is quoted on the Wikipedia entry for TS4, but note that the interviewer says "as well" (not mentioned on the Wiki page).
That is why SG/SB are saying it is a possible Wii exclusive. They definitely do not have a contact inside Free Radical, they are just getting info from widely available sources and passing it off as insider info.
I would be very, very surprised if TS4 was a wii exclusive. I expect it to be released on all 3 platforms like TS2 and Future Perfect were last gen (it may even be released on PS2 as well).
