mainly because the results would be undeniable
Would they? what if the network is being controlled by the goverenment it self? 51% attack would be an easy way for them to manipulate the results... Unless they use a 3rd party blockchain like ethereum. There is nothing stopping them to manipulate the results.
But could they manipulate it if it was verifiably decentralized and was open source? Of course, it would be way easier for a government to create a closed source permissioned blockchain, as that would give them way more control over their people... Generally, that is what governments want, more control. The 51% attack is a problem that would need to be solved, any blockchain used for voting probably would be very centralized, as there would be no incentive to maintain the ledger.
I really am unsure how this would work or how something like this could be successfully implemented. Is there any more information on what Japan did for this? Who maintained the blockchain? What was the incentive? What algo was it? So many questions.