It's not likely that the builder curved the top tube or purpose in this application and that small amount. I would first look inside the frame to see if the tubes are fully inserted in the lugs and that the geometry looks correct. If all that looks good, I would look very closely for other curved tubes like at the bottom of the seat tubes. I would check the alighnment for other clues of what caused the issue.

If the only thing that pops up is a warped top tube, it will need replacement. I have a machine built specifically to remove subtle bends but not in a hardened condition. I would also try to locate the exact location of the bend, approx radii and if any distortion in the tube that took place during the process of warping it.