by 'blockchain-based' they mean, a normal election happened with traditional voting, and then some of the vote counters uploaded the tally of the votes they counted to a private blockchain
"The process, which aimed to prevent voter fraud in Sierra Leone, was powered by Agora, a blockchain voting solutions provider. The company used a permissioned blockchain to ensure that recorded data remained protected while also being transparent to the stakeholders, all of whom were given permitted access to the blockchain in order to tally the votes.
During the process, initial votes that are recorded on ballot papers are to be counted by neutral observers. The procedure, being watched by Agora, is the key step to entering that data on to the blockchain."
