A college graduate at the 25th percentile makes $730 per week, which is still 13.5 percent more than the median high school grad.
Here are the numbers, reproduced in tabular format.
High School Grads
What Rampell is suggesting is that if the median student who had stopped after high school could have gone on to do as well as the 25th percentile of students who graduated college, then college would have been worth it. However, how likely is it that the median student who stopped after high school could have graduated college? College completion rates, once you get away from the top schools, are rather low. So a reasonable guess is that the median student who stopped after high school would have wound up somewhere in the bottom half of the "some college" distribution, and would not necessarily be earning more money.
I very much like the use of percentiles here. You can tell that the distribution of weekly earnings within education categories is approximately log-normal (the distance from the 50th to the 75th percentile is about twice the distance from the 25th to the 50th). This log-normality says that the average will be above the median and hence will not be a particularly helpful summary statistic.
Also note how large the variation is within education categories relative to the variation across categories.