It's one of two pieces of information he is claiming to have have learned. It's relevant as it narrows down the list of suspects somewhat.

Men and women generally have different mating strategies so I guess the suggestion it's a woman could help explain motive. At the very least it's another (hypothesised) datapoint about the perpetrator. Why shouldn't it matter?

If McAfee is correct in his analysis, it matters because it could help narrow down who did it.