When used by Men's Rights types and anti-feminists, it's polemic. What I mean by that is it is an argument used specifically to counter claims by feminists, and in that respect specifically it doesn't matter if it is true, because the original statement is made by feminists and by women's groups.
In other words the argument goes like, "OK you feminists are saying women don't earn enough money, but then you go around saying women account for 80% of consumer expenditure, so what the hell's up with that?"
Gloria Steinem for example has used this statistic and its quite common with women's groups, especially if they are to do with promoting women in business. As in, "Your business needs to be pandering to women because they make up 80% of consumer expenditure."
As others have pointed out it's not clear how you'd go about measuring this.
It's easy to find many more feminist / women sites quoting these statistics. Is the statistic true? A lot of MRM groups would be among the first to point out that feminists lie with statistics all the time. But this is not relevant to the polemic of using feminists own statistics against them in an argument.