It is worth noting that power isn’t necessarily a by-the-books concept. More often than not political power isn’t dictated solely by what is written on legal parchments.
Malfurion is a huge name that wields incredible power within the Kaldorei society because he’s one of the great heroes who saved the world and lead them down a path away from the dangerous arcane, shaping their entire culture. His druids would likewise have some measure of power by virtue of having literal magical power and their direct association with Malfurion.
That doesn’t make it not a matriarchy though. As far as I can tell Tyrande does, in fact, have all the legal authority and political power by virtue of how the government is set up. The Sisterhood of Elune are the leaders of the night elves as a political body and that Sisterhood is still exclusively female at the top ranks. They command the military as well, which is a massive force of political power.
So in a government sense Tyrande controls both the faith and the military, two cornerstones of the Kaldorei civilization. Through the military she also wields the most direct power in that she could, in theory, choose who gets her protection and who doesn’t. Through faith she could also, theoretically, decide certain individuals are no longer “graced by Elune’s favor” and effectively excommunicate them.
Basically, Tyrande has immense direct legal and cultural authority. She could become a tyrant very easily if she had the mind to.
Malfurion only has legal authority over the druids. The druids have authority within the Kaldorei society due to their magical abilities and their cultural significance, but I don’t believe they have direct legal authority over anything outside of the Circle. The one exception being the ban on arcane which kind of makes sense as druids replaced mages as the Kaldorei’s chief casters.
However I don’t think Malfurion has ever really been in a position where he could become a tyrant save maybe at the very beginning of the new Kaldorei society, and even then a cultural struggle between the Faith of Elune and the druids would almost certainly go to the long standing religious institution with direct authority over the Sentinels.
The only confusing bit is where Malfurion and the druids made and enforced the law concerning the arcane, but this isn’t necessarily druids exerting political power on their own. It isn’t like Tyrande wasn’t on board with that particular law or that the druids didn’t have the full support of the Sisterhood in enforcing it. Could the druids have passed that law without the approval of the Sisterhood? I don’t believe so.
Basically “Who has the power” is more complicated than “who has legal authority” and a society can still be matriarchal and have a male organization with a degree of power within it. What is important is that the very top of that leadership totem pole is exclusively female and that the all female organization has a greater sphere of influence than the male group. Which in the case of the Kaldorei seems to be true.