The arrow basically says that 14% and 16% are similar.
"As a rich high school dropout, you have a 14% chance of being in the top 20%."
"As a poor college grad, you have a 16% chance of being in the bottom 20%."
If you're in that ~15% of each group, you get the premise that the "poor" college grads didn't do better than the corresponding "rich" dropouts.
But that makes no sense--if you want high social mobility (and you think college grads should be paid more), you want both those numbers to be lower. The article treats it as a problem that they're similar.
"As a rich high school dropout, you have a 14% chance of being in the top 20%." "As a poor college grad, you have a 16% chance of being in the bottom 20%."
If you're in that ~15% of each group, you get the premise that the "poor" college grads didn't do better than the corresponding "rich" dropouts.