The story of Mulek served a very useful purpose even so—it allowed the people to merge, not with the hostility of conquerors over the conquered, though in fact that is what the relationship fundamentally was, but rather with the idea of brotherhood. They were all Israelites. Thus no one had any reason to question the Mulek story, because, while it failed in its original purpose, to allow Zarahemla to prevail over Mosiah, it still served the valuable function of uniting the newly combined nation as a single tribe.
In other words, The Book of Mormon here dramatizes a figment of man-made clap-trap religion being tolerated for its useful social function. Sound familiar?
What does it mean, then, that The Book of Mormon contains within itself the satirical claims advanced in "The Book of Mormon"? Not much, probably. Most literary criticism doesn't mean much. Parker and Stone might get a chuckle out of the idea. I have a feeling Joseph would, too, and maybe add a power ballade of his own.