My initial thought was that they were doing that with the DNA, but when I heard the evidence about it, it seemed to me that the sample itself and how likely it was to be Bradley ruled out Troy anyway.confirming Troy was in shark bay when SS disappeared. wonder if he was confirmed as alibied for the nights of the other murders also? possibly so as to limit possible challenges against an old and degraded DNA sample by saying it was the brother to create doubt?
I do think they may have specifically ruled him out for Sarah, though, because they had no forensic evidence in that case. I don’t think they would have wanted the defence to be able to put forward a theory that Sarah was killed by someone else specifically; wanted to restrict them to arguing it was someone else generally.
Perhaps also they were looking to establish whether Bradley had access to other properties (ie where his brother lived) or if Troy could be used in any way to establish an alibi for Bradley.