His testimony doesn't appear to be very reliable. At least according to this New York Times reporter

That should have been the end of it. Guede initially told one story: that he had sex with Kercher and then went into the bathroom, plugged in his iPod, and came out to find a strange man standing over her with a knife.

Then, months later, Guede changed his story: he said that strange man was now Sollecito, assisted by Amanda Knox in a sex game that went wrong. Neither of them had been named by him before. Guede denied being the killer.