The article covers how it started. If you are in currency-less barter only hunter gatherer society, it is not yet a medium of exchange. You would have better luck trading with food than anything else in those societies, as covered in the article.
Also diamond wasn't and still isn't used as a currency. Only in the last 10 years, it became a somewhat semi-serious investment grade material.
Yes and at some point you need a token of value. Imagine the plight of the itinerant tradesman as he herds his chickens through the forest from village to village simply so he can trade.
No no, food is far too cumbersome to be a token of value. Much better to slip a tiny token into ones waistband and exchange it in the markets. Gold was an exceedingly good token of value.
Diamonds are not currency because they are not easily divisible. But they have been a store of value for centuries now, and that makes them money.
Not as their main form of trade. See "Debt: the first 5000 years". That book also has an interesting discussion of the various theories about the origin of money, and the evidence supporting different ones.
Also diamond wasn't and still isn't used as a currency. Only in the last 10 years, it became a somewhat semi-serious investment grade material.