I took the quote [1] to basically mean "we might think this is a nuclear warhead, but in fact it is not, rather it is some kind of random test object used to demonstrate the software". Obscure part of a washing mashine, random geometric shape, etc.
[1] "The “obvious” answer, if my above assertions are true, is that it must not actually represent a thermonuclear secondary. [...] It could be some kind of pre-approved “unclassified shape” which is used for diagnostics and model verification, for example."