From FAFSA for 2016,
“Your eligibility is decided by the FAFSA. Students whose total family income is $50,000 a year or less qualify, but most Pell grant money goes to students with a total family income below $20,000.”
Median income in 2016 was $59000.
There are many families whose income is between $20000 and $50000 who will not get a grant, so saying that only 10% of the student population at WUSTL comes from families in the lower half is blatantly overstepping the data.