If those minimum wall thickness measurements are in the last third of the barrel then I'm not surprised it passed proof. The pressures fall off rapidly towards the muzzle. Most barrels I've measured seem the thinnest from just behind the chokes to eight or nine inches back. It's less than I would want but guns that thin do pass proof.

I have a gun that is 19 thou in a small spot about five inches from the muzzle. I've shot it extensively without a problem.