I think Reinhardt has the right view of the overall case here, but the 2nd Amendment comment is a throw-away. While it refers to (and thus acknowledges) the 2nd Amendment as a right, it doesn't enter into any discussion of the degree to which said right may Constitutionally be regulated. All he's saying is the obvious: "Carrying a gun is not prima facie evidence of intent to commit murder." That should be a truism, just as "Renting a U-Haul truck is not proof of intent to blow up a Federal office building."
As the OP mentioned, this is not a 2A case. I don't think the state courts (and the majority of the federal court panel) viewed carrying a gun as prima facie evidence of an intent to commit murder. It was simply a relatively minor factor which would allow any reasonable person to know that there was a substantial possibility the trigger man would end up shooting someone in an armed robbery in which the defendant actively participated. I've mentioned the other factors like the guy beating up a nurse, pointing guns at a person's head, and the armed robbery itself. Reinhardt's view of the entire case is wrong.
The federal court is constrained in what it can review. Did the state courts unreasonably apply settled U.S. Supreme Court precedent or did they unreasonably find facts. The U.S. Supreme Court reads this to mean "no reasonable jurist" would conclude what the state courts concluded. The very fact it may be debatable means the federal courts have to defer to state courts on this issue. As I mentioned before, Reinhardt is reversed in every single capital habeas case he authors. When he authors a majority opinion in this area of the law, it's just a red flag for the U.S. Supremes to grant cert and reverse.