Glad you got it sorted. Worth knowing that welded-in nuts like that often have weld spatter or paint/e-coat sitting in the threads, so it's not unusual on a brand new frame. A chaser was the right call over a tap since it cleans up the existing thread rather than cutting new metal.
Before you torque that bolt down, I'd put a dab of anti-seize or a little grease on it. That nut is behind the horn area where it sees road spray, so it'll be much easier to remove next time you pull the bars off.