That is the crux of the matter. I've wondered if they hadn't met earlier than Obama claims in NYC during their time at Columbia.
This is only a theory, but it seems plausible that Ayers and Obama left New York for Chicago (in about the same time-frame), Dohrn and Michelle were co-workers at a law firm in Chicago and subsequently that's when Obama was introduced to Michelle. That's just my idea for what it's worth.