The thing the author seems to be confused about is that when people talk about stable coin they aren't claiming that the market can't be irrational. They are claiming that over a sufficient period of time the coin will trend towards its target. In the case of DAI today, that target is 1 DAI == 1 USD. This means that while the market may irrationally buy/sell DAI for more or less than 1 USD, over time it will trend towards 1 USD.

For the vast majority of people looking for a stable coin, this is exactly what they want. They don't care so much that on a particular exchange DAI is worth more or less than $1, they care that in a week, or a month or a year DAI will still be worth "about a dollar" and not "100x USD" or "x/100 USD". DAI has the mechanisms necessary to make it profitable to drive the price of DAI back toward 1 USD if it loses its peg and those mechanisms have been shown to generally work in the wild so far.

I have personally made a few bucks when DAI transiently slipped its peg in the past. There are people actively authoring bots that profit off of DAI losing its peg and as those bots become more efficient and gain capital, I think we'll see DAI slip its peg less and less often.