Here is my simulation, for comparison purposes. I certainly would not dispute your statements.
You're correct that the snubbing network averages around 6W worth of heat dissipation.
I thought up another theory about the consultants' decision to use a snubbing TVS:
They may have designed transformer operation to create a predictable voltage range...
So that the TVS (a) activates only when voltage rises to a high enough level, and (b) conducts only briefly while voltage is above that level, and (c) carries miniscule Amperes when it conducts.
Hence they designed it so the TVS has an easy job to do. Perhaps it will not have its useful lifetime reduced.
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My observations about the turns ratio:
The 3.2 turns ratio causes the secondary to generate 131 V (420V divided by 3.2). This suggests the secondary has more turns than necessary.
If D1 were present, and conducting fully, tens of Amperes would come from the secondary during the 'on' cycle. However there is no diode in that position in a normal flyback.
My simulation shows the design can work. There is an advantage in having a 25% duty cycle, since there is adequate room to lengthen it if desired, to obtain greater output power.