IF AK and RS were indeed NOT present - I believe they were not, but cannot be 100% certain - then Guede would of course do them a good turn by admitting so.
Possibilities of why Rudy maintains his silence on this matter :
1. They really were there. (I do not think so, but it is possible although to me improbable.)
2. They were not there, but others were: Guede does not want to open up a can of worms about who else was. As Perugia Shock's Frank surmised, owing drug $ and bringing dealers or junkies in is a thing of shame to Rudy.
3. Guede was a lone wolf killer, and does not want to be viewed as such. His 13 years do not amount to much if he was. Easier to allow Knox and Sollecito to help buffer the blows of fortune.
Yeah, I mean here's the thing. If Rudy decides he wants to spill the beans he's going to make damn sure it doesn't backfire on him in any way. So what would that require? Probably a conversation with his lawyer that would itself probably be too risky to even try for a multitude of reasons.