http://www.catalyst.org/publication/256/buying-power
http://www.she-conomy.com/report/marketing-to-women-quick-fa...
Whether the top earners are men and the huge disparity on Bill Gates/Larry Ellison level wealth do not matter at all in this discussion.
Advertisers and companies are not competing for THOSE people's money (except if they make private jets and luxury cars), they want the money of the average household. 100,000,000 families have brought Apple products, for example, and all of the top 1000 wealthiest guys just bought another 1,000-10,000 combined.
So, when they say they control 51% of private wealth, that might or might not be true overall, but it is very true for the kind of wealth that matters to companies/advertisers. And it is even more true that women also influence what the family will buy, even more so than men.
Edit: It's probably not a good sign that they have two directly contradictory statistics for the amount of US private wealth controlled by women.