The author doesn't convincingly rule out the possibility that this is what it is. The other possible answers seem less plausible than it being a fake shape for software testing that happens to look fancier than past test shapes.
The author wasn't trying to be convincing. "I’m just surprised the DOE would release any image that gave really any implied graphical structure of a thermonuclear secondary, even if it is clearly schematic and meant to be only somewhat representative. It’s more than they usually allow!"
My reply was to point out that the author discussed issues related to HelloNurse's suggestion.
The tone of this kind of article has to be nice and diplomatic, and mistakes need to remain hypothetical and attributed to the largest possible organizational unit (DARPA having surprising policies, not the mechanical finite element simulation software team spreading data they consider harmless).
The second object that appears near the end of the article looks like a simplified version of the first with more basic shapes, as if someone was asked by someone else to draw a less suggestive replacement of the original (possibly with the sole purpose of appearing in slides); in a natural design process the cruder design would have appeared first.
