She should have apologized immediately once Sanders corrected her. “Immediately” meaning the next words out of her mouth, not “But you’re on some random list somewhere that must be accurate!”.
And the anti-Semitism in the statement seems pretty clear to me. The implication of “You’re an Israeli citizen…” is “…and you are therefore on the side of the Evil Jews”. There’s a reason that the list-maker only included politicians they don’t like, and why they picked Israel as the nation to have the dual citizenship.