What I got out of it is that Elune could have theorietically intervened somehow. Probably not in a grand blinding flash of light, but maybe in a more subtle way by maybe stopping Sylvanas somehow, or giving Tyrande or Malfurion a boost to prevent Saurfang from managing to attack Malfurion from behind.
What I guess we are supposed to conclude then is that Elune didnt intervene either because she distincly needed the Nelves to die for the greater good of saving Ardenweald (which form her point of view would prevent Zovaal from gaining power) or she couldnt/didnt want to directly intervene in what she might have assumed were mortal affairs. Elune didnt know Zovaal was plotting anything, and therefore probably didnt know that Sylvanas was his agent either, so from her point of view I guess she probably just saw it as a run of the mill war.
Regardless its a case where a big question was asked on why Elune didnt directly intervene to stop Sylvanas. And while I guess the reasoning works if you make some assumptions and stretch to reach conclusions there obvious outcome here is that Elune didnt stop Teldrassil burning because of a misunderstanding, which is horribly weak as a resolution goes.