To a large extent, theory is definition. A theory of literature is therefore definition of its many elements and how they work together to allow the creation of literature. And as far as I can tell, before Orson F. Whitney, few Mormons attempted anything near a theory of literature. A few definitions of elements of literature appeared here and there, but no one covered as many elements of literature as Whitney.
In the following extract, also from the 5-part article he published in 1926, Whitney discusses poetry, and after rejecting a common definition, he provides his own: