See here is the thing. At no point do the notes state that the Pantheon is actually the Pantheon of Order. They are still creatures of the Physical Realm. It's just that Order WON their attention and alignment early on. The Six Forces are at war. EVERYONE was likely trying to recruit the Titans in their way. The Void Lords very clearly are trying to do this; that's the entire plan with the Old Gods and the glee of Xal to get a foothold in the Seat of the Pantheon. The entire Nathrezim plot in Death lead to that; not just to knock the Arbiter out but then to create a new one powered by a Titan Soul.
My opinion is this. The universe is an experiment by the First Ones and the Titans are the desired end product. They set up six forces and then the physical reality for them to interact in. The APEX result of that interaction is the Pantheon (and above them all, Azeroth). Xal'atath suggests that the war between Void and Light and likely the war between all the forces has lasted far longer than the Pantheon existed.
When the Titans came to be the force that managed to get them on their side was Order. Some element of Order (possibly the Constellar) brought the Titans to Zereth Ordos, taught them the Purpose but through Order's lense and let them go.