The history is actually almost completely the reverse.
Just one example, from the executive summary of Cato Institute briefing paper #17 on the Davis-Bacon act:
“The Davis-Bacon Act, which requires that federal construction contractors pay their workers “prevailing wages,” was passed by Congress in 1931 with the intent of favoring white workers who belonged to white-only unions over non-unionized black workers. The act continues to have discriminatory effects today by favoring disproportionately white, skilled and unionized construction workers over disproportionately black, unskilled and non-unionized construction workers. Because Davis-Bacon was passed with discriminatory intent and continues to have discriminatory effects, its enforcement violates the Constitution’s guarantee of equal protection of the law.”
As you correctly point out, a know-it-all liberal once again gets things exactly bassackwards.
Of course it’s the reverse. What would he say about the Mississippi “Black” Code.