Not saying I believe the premise of this submission, as it seems pretty far fetched, but...
Say I want to verify that some anti-nuke weapon system can disable nuclear weapons. Say I've tested it to every extent possible, and now I want to verify its effectiveness against real weapon systems. Do you test it against the enemy and risk an actual nuclear war? Nope, you test it on your own weapons. The US has plenty, so one or two missiles at a time being disabled isn't going to be much of a tactical disadvantage, and it could be well planned in advance such that a real nuclear launch is impossible (by placing "real deal" missiles into silos, while subtracting the fissile material) in the case of malfunction as a result of your anti-nuke weapon system.
Unlikely, sure. But much more likely than the combination of aliens having made contact with Earth, the government having kept it from us, and the aliens having an interest in our nuclear weapon systems, as presumably species which can travel such distances would already have the tech to wipe us away and then some.