Through the wonder of Gallica, I went back to the original French, where they shot at buttes, which has the same meaning as the butts used as archery targets in English.
That was like scratching an itch.
There are some lesser differences as well. I read it as:
In this art young Englishmen are still instructed from early youth, and for this reason they continually surpass other archers. They shoot at the butts from a distance of six hundred feet.