Problem is, there's absolutely zero incentive for him to tell the truth now, if indeed he acted alone - and in fact there's a feasible incentive for him to stick to his story. It's easy to see that, despite his trial being over, he thinks he may be slightly better regarded as the "least culpable" of a gang of three which included Knox and Sollecito: he can still finesse in his own mind (and to other inmates/guards/friends(?)) that he was caught up in something that he'd never intended. However, if indeed he was there on his own, he has nowhere to hide, and nobody to hide behind.
I don't think we'll ever hear the truth out of Guede's mouth. He implicated Knox and Sollecito because it was clearly expedient for him to do so (and he knew they were in custody and the "case was closed" by the time he himself was arrested). By the same logic, he won't now come out and say that this was all a lie and that they weren't there.
So, in conclusion, I think it's not correct to say that if Guede were the sole assailant, he would now come out and admit that. Similarly, I think it's incorrect to infer that if Guede doesn't come out with a "sole assailant" confession now, this in any way means that Knox and/or Sollecito (or anyone else, for that matter) were in any way involved.