It’s quite simple, if Shane seen him in the house between 1700 and 1730 or when they had tea it could not have been Luke. If he could not confirm for sure he ‘seen’ Luke then all he need confirm is that tea was cook and ready when his mum got home. If Shane did not cook, his mum was at work then again Luke must have been home cooking the dinner and it could not be him.
Which is exactly why the police refused to accept the statements of the burnt pies. Looking back at the interviews, it's clear that, from the moment the Family Liaison officer stepped foot inside the Mitchell house, she was there to destroy Luke's alibi. She was telling Shane to "picture" things in his head just a few days after the murder - if that's not a blatant attempt to influence recall, I don't know what is. The trauma of the police treatment of Shane (in particular the interrogation on 14th April 2004) would have been enough to break any man - a point DF made very strongly at appeal. His treatment on the stand compounded that trauma - right up to the point where they thrust pictures of Jodi's naked, mutilated body in front of him (without warning) Shane was very clear that he
did see Luke that evening, but the police wouldn't accept his statements and kept "putting words in his mouth." After the shock of the photographs, he was forced to carry on giving evidence - any psychologist will tell you that a witness will become extremely manipulable immediately after such a shock because they can't think straight.
It is utterly disgusting that this sort of treatment is allowed. Having caused such shock and distress, the prosecution QC moved straight onto questions about masturbating to pornographic images on the internet - can you imagine trying to make any sense of that in those circumstances?
And, before the inevitable comments about him "admitting" that he was masturbating, he didn't. I have the court transcripts - it was put to him that he might have been "doing something else" whilst looking at the images ... Shane agreed that he "may have been" - that's it. We now, know, of course, that the "pornographic images" were almost certainly pop-ups, on screen for a few seconds, so he wasn't "watching porn" as has been claimed for all these years.
That said, is it any surprise that Shane just wanted to disappear and have the whole thing go away? It's not about believing Luke to be guilty, it's about self-preservation - Shane's life still had to go on out here after such intense media coverage and such personal trauma and humiliation.