Unuseless in this sense means not useless but also not useful, so a pointless less invention that actually does something, just not anything useful. The PMP cranks are actually useless as an invention, since they don't actually work or do anything. Chindogu need to actually do the thing they're supposed to do, but make you think the solution is stupid, so cumbersome or awkward as to not have any utility, or make you wonder why anyone would need to do the thing the invention does.
If you took a CPSC screw on a NR RD, but said it was invented to prevent you from not getting stabbed with the screw instead of meeting regulatory requirements, then that would be unuseless. It technically does the thing it is supposed to do, but is for all intents and purposes, useless.