It was clear from before and is becoming more obvious after the end-game cut-scene that Azeroth is not a Titan. However, Azeroth is ancient and probably much much older than the Titans. This gives us the indication that she is probably one of the First Ones and so was Argus. Azeroth was probably the First One of Life while Argus was the First One of Death.
We have seen in many fables that progenitors of opposite forces have been siblings, no matter how they became so. For example, Lucifer was an Angel who became the Devil (or the Master of Demons) who is also a brother to many other Angels - thus we see opposite forces of good and evil commanded by brothers.
Now in case of Azeroth and Argus, I can’t tell how they commanded opposite forces of life and death but they are probably siblings. That is why it is important to notice why Sargeras, supposedly feared by denizens of reality as the destroyer of planets/world souls, have instead harnessed the power of Argus. I think he knew who Argus actually was and how his power can bolster might of Chaos, Fel and ultimately the Legion.
Another leading theory from this can be that if we assume that Azeroth and Argus were the First Ones of Creation (Life) and Destruction (Death), there are also probably the First Ones of Chaos, Order, Light and the Void. Which brings me to another assumption: Aman’thul is also not a Titan. He is the First One of Order and in his quest to establish order in all of cosmos, he made powerful allies from different dimensions and named them as a Group called The Titans.
Pretty Wild huh? Let me know of what you think, brothers and sisters.