The Ivy Mike device (apart from the primary) didn't sound like it required huge precision - I'm sure "Dark Sun" refers to the radiation case being filled with expanded polystyrene held in place by copper nails hammered into place.
The Ivy Mike device used cryogenic liquid deuterium instead of solid lithium-6 hydride, so the secondary was very different from any sort of practical hydrogen bomb. Maybe the tolerances were looser for the Ivy Mike device, maybe not, but I wouldn't draw many conclusions from it.
Ivy Mike was also a much larger experimental apparatus. In addition to liquid deuterium cooling, it's possible the designers used other mechanisms to relax tolerances such as.
1. Using a much larger fission yield then necessary
2. Using a much larger/denser elispes than necessary to avoid manufacturing errors becoming problematic.
3. Using a much more massive reflector than necessary.
The goal of the Ivy Mike test was to validate the concept of the design - not necessarily to validate that it could be made into a practical bomb. I'm not sure if its ever been revealed wether the filler for the shell has any significance towards the final device.
Examples.
- initiators in fission devices
- explosive lenses in fission devices (what’s the exact mix/shape/required explosive payload?
- large scale manufacture of high grade plutonium. What’s the exact effect of plutonium purity on yield/function?
- how effective of a reflector is required for the teller ullman design? What’s the effect of manufacturing error on the ellipses?
There are more, but any one of these questions would require time, money, and expertise to resolve.