They didn't announce any physical findings of the autopsy. If Breitbart had suffered an acute myocardial infarction, they would have in all likelihood seen something, and they probably would have reported it because such a finding would quell any suspicion of foul play. Same holds for most of the other common natural causes of sudden death.
We probably won't know much more for weeks, if even then. Wonder how much political pressure is being applied at the coroner's office. This is a Democrat-controlled jurisdiction, after all.