Where does the US Department of Education report university graduation rates?
I can't find a particular report that contains your exact quote, but the statistic could easily be derived from data that is present in the "Digest of Educational Statistics" published by the US Department of Education's National Center for Education Statistics:

https://nces.ed.gov/programs/digest/

In particular, see table 326.40 in the "current" edition which gives this percentage separately for students who started in 2 year and 4 year institutions and for students from families in various income ranges.  To get the 40% statistic quoted, you'd have to combine the rates for students in two income ranges and the statistics for students starting at 2 year and 4 year colleges.  These range from 17.6% (for students from families with income between $45,000 and $69,999 who started at 2 year colleges) to 66.7% (for students from families with income between $70,000 and $99,999 who started at 4 year colleges.)  

The statistic shouldn't be surprising. Keep in mind that not everyone who enters "college" is seeking a bachelor's degree- many will want only an associates degree or certificate of some sort.  If you limit your attention to students who enter four year colleges (and presumably want a bachelor's degree) and who come from higher income families ($100,000 per year and up) you get a much higher rate of 77.5%.  Both family income and starting at a 4 year college are important factors.