My responses, based on my current understanding, such as it is.

1. Note the qualifier "marketplace clears". In the process of clearing, a market undergoes price-discovery and any arbitrage opportunities are taken and thus removed.

2. In a pre-monetary, fledgling economy, there could be two or more commodities which simultaneously serve as mediums of exchange. What makes one of them the choice as Money is that the economy grows to a point where there is significant surplus wealth to be stored and one of the commodities wins that competition. The win may have nothing to do with any unique attribute - randomness could easily tilt the competition a certain way. In an economy where people have generated and thus wish to save their surplus wealth, one dominant money will arise (because of the winner take all effects described). And it will be one of the mediums of exchange from amongst which the store of surplus value will be chosen. Fiat money is a good, in economic terms. It exchanges for other goods. It is currently Money cos it is used by people to store their surplus.

3. The purpose of the article is not a detailed historical account. It is an explanation of sound monetary economics. It's purpose and its value is in the deduction. The full history of monetary evolution is not germane to the deduction.