Had they agreed with the argument, they had a duty to say so. They are in the business of providing decisions that lower courts can use as FINAL legal guidance in the case at hand, in so far as they have the information to do so.
To suggest that the militia membership of Miller and Layton was going to be an issue at some later time is to suggest that the legal system works other than the way it does.
But they had no such duty if they disagreed?
"To suggest that the militia membership of Miller and Layton was going to be an issue at some later time"
The court didn't even know if this was a Militia-type weapon! Why in the world would they concern themselves with Militia membership?