Julie would know Bamber was involved after the previous 18 months.
It admits indirect involvement as there was no point in him denying it.
He wanted to boast about his brilliant plan, but distance himself from actually being there.
He knew Julie was not not going to see MM in the near future.
It shows he is not so bad as to carry out the massacre himself. Which may deter her from going to the police.
It will highlight Julie being wrong if she tells the police MM did it. Which will undermine her whole accusation. This is what happened.
If MM did it, Julie would be accusing two people. Who would both deny it and cover for each other. This would deter Julie from approaching the police.
If MM was involved, there may be other people involved. Who would cover for each other. This would deter Julie from going to the police.
Julie may believe MM did it, as he had a reputation of a mercenary.
Julie may believe MM did it, as the alleged 2k payment was not impossible to acquire.
If MM did it, there would be no forensic evidence linking Bamber to the crime. A jilted lover seeking revenge may only end up convicting MM. Or neither.
If MM did it, Bamber may have an alibi. He could easily get an ex girlfriend to stay the night with him.
If MM did it, it would mean Bamber was at home all night. Which he may be able to prove. No one would have certainly seen him leave his cottage.
Julie may go to the police anyway. Based on what Bamber had told her pre massacre. Julie doing this and then bringing up MM, undermines her statement.
Julie asked him. He gave an answer.