Thanks to all for the replies. I did write it up as being most likely a non-listed component and I recommended an assessment and repairs as necessary by a licensed HVAC installer -- and not the same one that installed it! As I said, the clearance to combustibles was not an issue. That was all clearly visible from the attic. The furnace was easily accessible as was the B vent up to the roof--you could walk right up to it.
The strength and wind resistance of the assembly wasn't an issue either. The vertical section had tabs forming a flange and they were carefully folded back and soldered to the underside of the base, as well as around the perimeter of the opening on the top side. The base was made to the exact slope of the roof so the vent stood plumb.
I can't imagine anything short of a tornado tearing it off, and any other conventional flashings would be gone at that point also. Along with your turbine vents and everything else.