Usually things like this are set up beforehand, and they may be here, especially considering the intricacy of the song. But if that's not the case, it would be for a few reasons. First, she is famously shy with doing interviews and live performances, and her initial cry of "No!!!" seems pretty real. And second, afterwards, he acknowledges not having been strong on all the words, which likely wouldn't be the case if it was rehearsed.
Whatever the truth, it's a totally charming performance. All the more so if it's impromptu with someone so shy at such things.