That's covered in the blog post:

First, that archers can actually be more effective against mounted troops than foot: the mounted troops ride close together, horses are hard to fully armor, and one horse getting hit in the leg can cause a lot of chaos.

Second, at Agincourt, the French knights _walked_ through the arrow-fire quite successfully, but the effort (physical, mental, cumulative effect of small wounds) tired them enough that the English soldiers could beat them hand-to-hand. And that this ability to inflict small damage before the main fighting is why archers were valuable.