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 to…
It may attempt to trend towards the dollar over time, but will it be trending that way during the time that you personally need it? "The market can remain irrational longer than you can remain solvent", springs immediately to mind. edit: Also, pegging a cryptocurrency to the dollar seems almost comedically perverse. What is it for?
The general thing you want is for your currency to have stable purchasing power (for goods and services) in the short term, maybe inflate it slowly to encourage investment - monetary policy basically. Targeting a USD peg is a way to do this, of course ideally you'd want your currency to have a monetary policy independent of USD