Come, mighty not-might

Continuing today's observations of English usage in newsprint, I notice the following passage in a discussion by Anthony Lewis in today's New York Times:

It was as profound a day in the court as any in a long time. The justices did what they have often shied away from doing: said no to the argument that the title commander-in-chief means that the president can do whatever he says is necessary to win a war. In 1944, for example, the court upheld President Franklin D. Roosevelt's order to remove Japanese-Americans from their homes on the West Coast and confine them in desert camps — on the thin argument, as it turned out false, that they might be disloyal. (my emphasis)

I'm curious about that use of 'might'. It takes a lot to negate a 'might'. What does Lewis mean when he says that it was false that Japanese-Americans might be disloyal? (Set aside the difficulty with an argument having a truth-value.) He seems to assume false an epistemic view of 'might' -- the idea that "they might be disloyal" means that "I don't know that they won't be disloyal". I take it it is uncontroversial that FDR & co. did believe that the Japanese-Americans might be disloyal. If this is correct, then an epistemic reading of 'might' makes the sentence in question true. And he obviously doesn't mean this to be a rigorous modal necessity statement -- there clearly are possible worlds where the Japanese-Americans were disloyal. I am, of course, not doing anything close to defending the U.S. policy. The internment of Japanese-Americans is one of the darkest stains in recent American history (and autobiographically, I may as well add that my own family, including my grandparents, was in an internment camp). What I'm saying is that the problem with the internment was not that it failed to match the officials' beliefs, but rather that it failed to match beliefs that it would have been reasonable for them to have had. So what do we do with Lewis's 'might'? It seems as though he intends it in a particularly normatively epistemic sense -- "They might be disloyal" means "It would be reasonable for me not to believe that it is not the case that they won't be disloyal". I'm not sure how I feel about that reading -- it might be semantically plausible. Or maybe he's just misusing the word. It did sound a little funny when I read it.