The previous discussion is about full validation. Libbitcoin validates blocks (prevout lookup an script execution) concurrently, which is a significant performance advantage. However, in milestone runs there is no need to look up prevouts.
You might believe that it’s the lack of this lookup that saves the time, but it’s not. It’s the lack of a utxo store imposing a total order on block commitment. This allows us to download, check, store and index blocks concurrently across the entire chain.