Teaching “after this manner” can indicate either that many teach this philosophy or that many teach some version of this philosophy. This second seems more likely to me, because Nephi is making what I interpret as a hyperbolic statement for effect; nevertheless, variations of this philosophy that acknowledge but minimize the effects of sin are extremely influential.
Reference: Nephi is restating Isaiah 29:15: “Wo unto them that seek deep to hide their counsel from the Lord, and their works are in the dark, and they say, Who seeth us? and who knoweth us?” However, Nephi is no longer constructing a pesher-style commentary but is rather making a literary allusion—evoking a familiar phrase as he continues his discussion. This technique is like the jazz performer who plays the opening theme from “The Sound of Music” in the middle of his own improvisation or the comedian who answers a question about his age by intoning, “Fourscore and seven years ago.… ” Each assumes that the audience will recognize the quotation and see the juxtaposition as (in this case) comic. Similarly, Nephi is simply using a passage familiar to his audience that has associations of tone and effect similar to those he wants. He is not using Isaiah as a proof text or making a commentary on Isaiah. Such unexplained allusions are typical artistic techniques in many high-context societies. (See Behind the Text: Chapter 1, “Text and Context,” for more information on high-context cultures.)