I've never thought anyone in the semifinals to finals cannot sing.Some sing better than others, to be sure, but I think they all can sing.
The problem is can they sing when the pressure is on. Do they have the technique and experience to sound good on stage in front of millions of people. Do they know how to protect the voice so they sound good consistantly and don't strain their voice in practice, etc.
Or do they let the nerves get to them which makes the voice a bit small and strangled.
Being able to perform is a separate skill set from being able to sing.
For instance. Singing just for pleasure I do OK, In a performance I do OK. Can't sing in an audition to save my life. Seems my voice does a lot of the same things at an audition that it sounds like I said above that Sanjaya's is doing on performance nights. If I really wanted to sing on stage I'd have to get a coach that could get me over the audition problem and give me enough technique for sustained used of my voice.