Lot of the advice in the article would fail in some situations like the one you point out.

Unless the quantity is less than 5, tap to increase doesn't make sense.

Also, the slider wouldn't work in case you are trying to choose from an even average sized list on "min" or "max" side. There are only so many pixels in portrait mode and my big thumb - how can you expect it to be so precise. While in case of the dropdown in almost 2 scrolls i can get to the exact number by varying the speed of scroll, like binary search.

Some of the points are correct though, i don't see many people making those mistakes.