Okay, I see what you're thinking. I don't think they meant "knew her" as in before that night necessarily - just that her story about picking Mangum up as a complete stranger was a lie. All they figured out is that the woman who dropped her at Kroger's was also at the lax house to dance and "knew her" as in not just picking up a stranger along the road, which was what Kim said.
More good stuff from Liestoppers.
http://liestoppers.blogspot.com/2006/09/hair-analysis.html