I don't believe this statistic is from girls that only play high school. Most also play TB, at least where I live they do.
As pattar mentioned, the 6% statistic is misleading because that's looking at HS players, but not all HS players are also TB players, so it's a distorted picture. Looking at the percentage of 18U TB players who go on to play college ball is a more accurate statistic, especially since that's where the majority of time and money is spent. I have not idea what that percentage it, but I would bet it's a pretty high number.