The examples in the article were merely illustrative.
In principle you shouldn’t mix the content of .btn with the .card__button. This second should contain all the CSS specific to the buttons on cards and if you want to have different styles of buttons use the modifier.
So, for example, you would have:
.card__button ← General styles of buttons on cards.
.card__button — small ← A smaller button on the cards
.card__button — big ← A bigger button on the cards