The elemental aspect of CNJ lore goes well beyond the fact that they have elemental nukes. The guild literally defines what a CNJ is as the physical extension of the elements in your person. The CNJs speak to and deal with elementals as a critical part of what they do on a daily basis. Sure, every spell has an element, BUT those elementals don't come in astral and umbral, only the 6 elements of nature (fire, ice, wind, lightning, water, earth), matching CNJ nukes and the elements that BLM historically uses.
As for THM, they worship the god Nald'Thal, who deals in life and death. The THM guild is stationed in the Hall of Thal, and Thal is the half that deals in death. WHM, once introduced, can provide the balance THM needs, by filling the role stationed in the Hall of Nald, and taking up the half that deals in life. This completes the balance that THM is lacking in their lore.