I think it was just a DUMB but HONEST mistake.
I dunno. Maybe she should have known better, but honestly, I don't believe this story amounts to a hill of beans.
In fact, while I (in general) agree with the appropriate limitation of time/place on this kind of speech, I am not really convinced it is all that applicable under the circumstances.
In short, the First Lady may have made a boo-boo, but imho, this one should be promptly forgotten as inconsequential.