The analysis has a huge gap in that it looks only a pay differences and not at employment differences. The pay gap is large (37K vs 71K) [1] and the unemployment gap is over 2 percentage points [2]. Being unemployed for 6 months wipes out most of the nominal 'gain' by investing your tuition in equities. (and of course it depends on when you invest etc etc).

[1] https://www.bls.gov/emp/ep_table_education_summary.htm

[2] https://www.bls.gov/emp/ep_chart_001.htm