There are many nods to the two groups being completely different, and basically the only argument some “thinkers” have about the First Ones being the Titans is that “order is imposed on the Shadowlands with the previous ruler being chained (the Jailer)”.
While the Titans did, in fact, change the status quo in places like Azeroth by imprisoning the previous masters of the planet (the Old Gods), we must take some things into consideration:
- They chained/imprisoned the Old Gods because they could not kill them, not because they did not want to (9.1 spoiler here, the Primus expressed hope that his fallen brother would attone for his sins).
- Imprisoning or killing someone to change the status quo is what everyone would do, not just the Titans. Plus the Light also forges/uses chains.
- Just because there is “order” with this robotic Arbiter, as her Honored Voice states, that doesn’t mean there was not some form of order before.
- The existence of order somewhere doesn’t mean that Order (capital “O”) has intervened there. The Light also has some form of order within its ranks and forms, while the Void doesn’t. What? Did the Titans invade the Light, or did the Legion impose disorder upon the Void?
Beside these things surrounding the Jailer’s imprisonment and a change upon the Shadowlands, there are these visible -intentionally visible- differences between the First Ones and the Titans:
- When we escape the Maw at the start of the expansion, Anduin states in the quest text that the shapes and inscriptions on the Waystone to Oribos seem unrecognizable to him, and made by a differend hand than that of the Titans, with Anduin claiming he has studied the Titans’ works/culture.
- When you zoomed out Azeroth in the in-game map before the Shadowlands expansion, you would see Aman’thul shrouding Azeroth with a protective veil of clouds, as if hiding the horrors of the Great Dark Beyond from its denizens. After the release of Shadowlands, Aman’thul and Sargeras are gone, the veil of clouds is lifted from Azeroth, and on the edges of the paper/map you can see Oribos-like markings.
- The architecture of Oribos resembles Ny’alotha more than any Titan building. It is just original, get over it.
- On the decorations of Oribos as well as on Ardenweald’s eternal charge, we can see that the First Ones value the “Cycle” between life and death. The Titans do not value the cycle- in fact they break it (see Odyn’s interventions on the dead Vrykul and keep in mind Eonar’s title: “the Life-Binder”).
- If the Titans had ordered the Shadowlands, why wouldn’t they inform their keepers like Odyn about what happens when mortals die? Odyn had to make a deal with an unknown-to-him being to see what is going on.
- In the fabled book “Enemy Infiltration- Preface”, which is a report written to Denathrius by his agents and children, the Dreadlords, the Unseen Guests report on how to defeat the Titans, among other cosmic rivals. This shows that the Titans are not the First Ones, since we know that the Eternal Ones, like Denathrius, know the First Ones and are their creations/children. That being said, Denathrius would not seek to kill the First Ones, nor could he, being much weaker, and the First Ones in turn would not be so blind, arrogant, and foolish to be fractured so easily.
- It has been stated in Blizzard interviews that the First Ones are connected to “various pantheons” and are the closest to a “true deity in Warcraft”.
And besides all the obvious evidence, it would be underwhelming and boring for the First Ones to turn out being the “good ol’ Titans” (that aren’t much strong, really).
I hope some particular speculators on YouTube keep these in mind, because, at its current state, Blizzard may completely ruin its own story for the sake of fan-service. An example of Blizzard’s risky decisions is the path they are taking with Elune in 9.1 (spoilers). Now, the Elune affair is still concealed in mystery and there is still some space to “save it”, but you just never know.