As of Legion we "know" MU Gul'dan only raised parts of it, but it nonetheless begs the question: How did he raise the islands?
He is a warlock proficient in death (creating death knights in example), fel and shadow magic, none of which are directly capable of performing such a feat to my knowledge. Of course he could have bound some great elemental (as he did to create his volcano), or perhaps struck a deal with another creature with such power (Deathwing comes to mind, but i have seen no indication that it was him).
The thing is: I do not know for certain.
But it also raises another question: To what extent are the magics of Warcraft versatile? Of course within their own realms they can freely shape reality, as seen extensively in the shadowlands, but in normal reality such fantastical non-pewpew natured magic seems mostly limited to the arcane; think of effective telekinesis, transportation, creation, etcetera, as well as, of course, magics that are directly relevant to whatever you wish to shape; nature and elemental magic chiefly.
Can fel/shadow/death/light magic be used to i.e. raise islands directly? Or is it more constrained to its "stereotypical" behaviour of damaging, healing, corrupting and the like?