To include the whole passage you are talking about: > The granddaddy of all consensus mechanisms—behind Bitcoin, Litecoin, Monero, and (for the time being at l

To include the whole passage you are talking about: > The granddaddy of all consensus mechanisms—behind Bitcoin, Litecoin, Monero, and (for the time being at least) Ethereum—is called proof of work. Essentially, PoW makes adding transactions to the blockchain computationally—and therefore financially—very expensive, so as to discourage fraudulent activity. At the same time, users who go to the trouble of creating valid blocks, known as mining, are rewarded with cryptocurrency. I would say this passage gets the facts totally correct. The author doesn't say that Proof-of-work has to do with the validity of transactions, they say it has to do with the validity of blocks. Indeed, it can't be easy to make valid blocks or double spending would become a problem as you say. Not sure why you think this reflects the misunderstanding of the inherent costs of consensus systems.