It isn't the prosecutions job to decide what is/isn't relevant, or what may/may not be viewed as exculpatory prior to indictment.
That's sort of the courts/jury's job. Now if a judge rules that it was relevant, and withheld, this guy could possibly get off on a legal technicality.
An autopsy would have been performed on the Gaudreaus due to the circumstances of their death, and the criminal investigation, and I find it hard to believe that the prosecution wouldnt have had the full results before going infront of a grand jury.