The fed law only covers you while traveling through, not conducting local business. He was not on military orders, he was traveling on his own. Also, having the guns on Walter Reed’s property is a huge nono, he knew better.
Like I said, I do not agree with it, but the gun laws, especially in that part of the country, are intended to trap people.
A fellow at one of the newspapers here once identified all the various police agencies in DC and defined all the spaces they had jurisdiction over.
Sometimes the jurisdictions conflicted ~ one such place was the Carillon near the Capitol. There were something more than a dozen types of cops had some sort of jurisdiction there and yet muggings were frequent!
Whatever goes on at Walter Reed is between the traveler on the property and provost marshall. You leave that place you are traveling interstate again.
We have this situation down here where you go to the town of Quantico VA to the gunstore THROUGH Quantico marine base ~ you actually have to stop at a couple of stop signs to make this trip.
I've done it several times and each time I had firearms in my vehicle. The Constitution protects my right to travel on that road which is actually part of the base ~ and hundreds of people each week do the same. The SPs could stop me or anybody and do whatever they wanted, but properly packed away and stored they really can't do anything about the firearms. And think about it, in this case we are not actually involved in INTERSTATE TRAVEL since these trips start and stop in Virginia.
The FBI and CIA agents who go there for training carry their own personal weapons on and off the base, and may actually go to restaurants in Quantico VA ~ whether they are on duty, off duty, or whathave you, the rules on the base are one thing, and the rules in Virginia are another, and the situation the soldier faced in DC never arises because the Virginia cops and the DOD cops know the rules.
Most bases won't let you carry weapons, but they do allow you to bring an unloaded weapon and the ammo and drop them off at the armory until it's time to leave.