“And he preached many things which were flattering unto the people; and this he did that he might overthrow the doctrine of Christ.”
At first glance, there is some tension in the statement that Sherem preached flattering things to the people and the fact that he vigorously championed the Law of Moses in his encounter with Jacob. Surely, belief that one is obligated to follow the rigid purity code of the law of Moses is not particularly “flattering” to the people?
The key might come in Sherem’s declaration that there has never been and will never be a Christ. Inherent in this statement is the belief that there is no real need for repentance or an atonement.
So even though Sherem on the face of things preached the Law of Moses, in reality he preached a consequence free form of it. Sure, you should try to keep the commandments, but there isn’t any real punishment for breaking them. And anyone who says otherwise is just moralizing or making things up.
So Sherem’s hearsay in truth resembles the false doctrine that Nephi warned of: “And there shall also be many which shall say: Eat, drink, and be merry; nevertheless, fear God—he will justify in committing a little sin.” If God doesn’t actually enforce the law, then mankind is under no real obligation to follow it. And commandments are little more than suggestions.
So this teaching would indeed be highly flattering and pleasing to the listener.