The explain box for 'chapeaux' says: "Plurals in French are generally formed by adding -s at the end of the word."
Yet chapeau is made plural by adding an -x? What gives?
The rule provided by Duo is accurate.
In the "explain box", they unfortunately have not enough space to describe the detail of exceptions:
One of them is: nouns of which singular ends in "-eau", take an -x in plural.