I would agree with you under "normal" circumstances, but this is not normal.
First, of all there is a difference between the words Michelle used, "home country," and your word, "homeland." In my experience, it's quite ununusal -- in fact, I have never heard it -- for a person to use the words "home country" to describe the place where their forefathers came from.
Secondly, this was in 2008 and it was spoken by the wife of a man who was running for President of the United STates. Say what you will about Michelle, she is no dummie. No political spouse, but especially not one in the environment the Obamas were in in 2008 and one with a husband with such a mysterious and unusual personal past, would stand up and start talking about her husband's "home country" without realizing that, whoops!, she ought to watch her words.
Therefore, one has to wonder why she did not. I mean, in the political environment they were in, even if she were to talk about Obama's "homeland," in the sense you suggest, that would be just stupid. Does she really have no clue that people might object to a President who feels he has a "homeland" or a "home country" other than the United States?
That was near the end of the campaign. They were very tired. That’s probably why she slipped and said it. Maybe in her mind, she thought, “oops” but decided it would probably go unnoticed and if it didn’t I guarantee you she has a response ready and waiting to cover for it. Just Like Stephanopolis covered for Obongo when he said “My Muslim faith”.