For plenty of reasons. One, it's hyperbolic in the extreme. Two, from where would someone acquire a nuclear weapon? Three, we signed Arms Control treaties limiting such things. Those treaties require the same number of votes to pass as an amendment and also are held in the same standard as an amendment (see Article 6). Four, who would sell them to a civilian in the first place, even if they were still being made today (they're not)? Five, they have no military utility and are therefore not protected by the 2A.
Are those enough reasons why it's a ridiculous and stupid argument?