The Betterton quote is actually a translated passage from Chaucer’s “The Reeve’s Tale:”

Til that hir corn was faire and weel ygrounde.
And whan the mele is sakked and ybounde

Johnson states in the Preface, “But as every language has a time of rudeness antecedent to perfection, as well as of false refinement and declension, I have been cautious lest my zeal for antiquity might drive me into times too remote, and croud my book with words now no longer understood. I have fixed Sidney’s work for the boundary, beyond which I make few excursions.”

By using this translation, Johnson is able to get Chaucer into the dictionary without using antiquated language. For more on this, see William Snell’s “A Note on Dr. Samuel Johnson and the Reception of Chaucer in Eighteenth-Century England” (Hiyoshi Review of English Studies, No. 44, 2004), available here.