Maybe I'm just not up on my history, but is it well-established that this letter changed anyone's mind? Or was it just very inspiring for people who already agreed with him? (The latter is, after all, the purpose of most political oration.) In any case, when Atwood says "It is remarkably persuasive without ever resorting to anger, incivility, or invective" he really seems to base that on reading the letter rather than an historical data.

Also, who would ever think that anger, incivility, or invectiveness would persuade?

This was only 3 sentences in Atwood's post, though, so it's not that big of a deal.