I would have a few quibbles with the article, for example the only daylily actually scientically shown to have "anti-freeze" is Hemerocallis fulva, which does not have blue-green leaves. That doesn't mean that others don't but it does mean that it isn't just a "blue-green" thing.
However, the last paragraph is incorrect, although a common misconception.. A deciduous plant has all the leaves die annually at one time, they don't necessarily have to fall off at that time for the plant to be considered deciduous. Consider oaks and beeches, which keep the dead leaves during winter until they fall, often lasting into spring. The word "dormant" means not growing, it does not mean dying leaves. A plant does not have to have its leaves die to be dormant. If you look around now you will see evergreen plants like conifers and rhododendrons that are dormant but still with green living leaves, and you will see "marcescent" plants that are deciduous but don't drop the dead leaves immediately, like this ninebark:
Yes when daylily leaves have died back and are not growing the plant is dormant, but then so are the evergreens and sevs where it is cold enough to stop growth. Deciduous actually better differentiates the plant behaviour, and "dormant" is not a foliage habit. Deciduous is what indicates that the leaves die every fall.
Edited to correct typo and clarity and add this picture of a dormant spruce tree taken today:
