Yeah, I went back and forth whether or not to use the terms basilect and acrolect. They’re not quite right. But AN was a prestige language.
Yes, English was a fully developed literary language by Langland’s time—it had never really ceased being such, especially outside of London. But Langland and Chaucer were riding the first wave of the English’s resurgence as a literary language in the fourteenth century. Data point: the first (surviving) post-Conquest literary collection written entirely in English is the Auchinleck manuscript, copied in the 1330s.