He added that he heard Amanda. She was there, she confessed to it so it would be rather strange if he hadn't noticed her in the cottage. Raf was already arrested as well and maybe for some strange reason he expected him to be found guilty so he might as well describe him in vague terms and not a 7 foot stranger.
It wouldn't make him very believable if he suddenly recognized the stranger (i am not saying his toilet story was credible but that is the card he played) and at the same time you push Raf in a corner who might just confess what really happened and explain Rudy's involvement. What good would that do?
Neither of them goes straight into identifying the others. They all admit a little and try to fool the police. Raf says Amanda could have done it since she left his house at 9 and came back after midnight. Amanda says the black guy did it but gives him a different name. And Rudy says Amanda was there but the Italian stranger did it. All liars, and all guilty as hell.
Righto! Thanks, sherlockh.