They are mistaken.
Section 10. No state shall enter into any treaty, alliance, or confederation; grant letters of marque and reprisal; coin money; emit bills of credit; make anything but gold and silver coin a tender in payment of debts; pass any bill of attainder, ex post facto law, or law impairing the obligation of contracts, or grant any title of nobility.
If ex post facto applied in civil matters, cotton1706, then bILLclinton could not have made his 1993 tax hikes retoractive to Jan. 1, 1993, 20 days before he began his presidential "role."