Ah, old Irving probably could have worded that section a little more clearly. It's really the presumption of innocence that's an argument from ignorance. We presume that the accused is innocent until proven guilty. But that presumption is itself an argument from ignorance - we don't, in fact, know that he is innocent, but we presume him to be innocent until proven otherwise. And insofar as that presumption may carry through to an acquittal, that acquittal may itself be an argument from ignorance - if the defendant does not present an affirmative defense to establish his own innocence, but is acquitted by virtue of the fact that the prosecution has failed to bear the burden of proof beyond a reasonable doubt, we still don't know that the defendant is, in fact, innocent - we simply default to an acquittal in the absence of evidence to the contrary, whether that acquittal is actually true to the facts or not. And in that case, the "not guilty" verdict is roughly equivalent to the verdict that the Scots have, but we don't - a verdict of "not proven".
But regardless of whether the verdict is a logical fallacy or not, we agree as a society to sacrifice a certain amount of logical cogency in the name of justice - as the authors paraphrase, we agree that it is better that 100 guilty men should go free than that one innocent man should be hanged. And in pursuit of that end, we agree to treat our ignorance as though it were, in fact, an affirmative conclusion, even though it isn't.
However, The principle of double jeopardy suggests to me that the law, in fact, reads a "not guilty" verdict as it stands, not as the Scots would have it, or as in "not proved guilty". I think US custom on this matter is morally correct. If the state, with it's virtually unlimited resources, chooses to bring an individual, with limited resources, to trial, and fails to make it's case, the morally correct conclusion, in my opinion, is "not guilty".