I take him at his word here:
https://www.youtube.com/watch?v=bMUgNg7aD8M
One of the only times he was telling the truth IMNSHO.
The Obama apologists claim that he just misspoke. I do not find that line of reasoning convincing at all.
As an analogy, I myself am not a citizen of Norway, and I never was one. So how could I possibly slip up and say “I am proud of my Norwegian citizenship.” Such a mistake is simply not possible.