The null hypothesis would be this: Lorca is pretty much a typical writer of his day and age. Typical in his class origin, in his formation, in his interest. He took an interest in popular music, folklore, and Flamenco, like Menéndez Pidal or Manuel Machado, or Rafael Alberti. He was interested in Góngora, like everyone else of his generation, and went on to write quasi-surrealist poetry, like Larrea, Cernuda, Aleixandre, Alberti, and the surrealist group of the Canary islands. He wrote neo-classical sonnets, like other poets of the early 30s. Like many poets, he wrote plays too. The Machado brothers, Alberti, even Unamuno. Nothing makes him stand out from the crowd, a crowd of admittedly brilliant poets and playwrights, except for his charismatic personality. Only his violent assassination at the beginning of the war raised him to another level. That symbolically resonant death and fact that his work conformed superficially to what most interested foreigners in Spain--the Romantic, Hemingwayesque version of Spanish cultural exceptionalism, made him popular in the English-speaking world.
The null hypothesis is useful in that it sets the bar at a certain level. We cannot just assume that Lorca is an exceptional writer. It also contains a good deal that is true. We cannot deny the multiple ways in which he seems to be one of many.
The refutation of this hypothesis would look at the distinctive achievement of his plays. Nobody else raised the theater to that level. He is comparable only to the greatest playwrights of the "Golden Age." Next, at his poetics, the complexity of his writing about poetry. Finally, to the fact that he was at worst primus inter pares, at best, the most talented poet in Spain since Góngora. Not to mention that once he was raised to a certain level, even by accident or mistake, his work becomes hypercanonical and hence attract endless hermeneutical events. Song settings, novels, false translations, etc... So it becomes impossible simply to view him as a normal poet of his age.