From hearing what Archivist or Conan says in dialogue, it probably can be true.
Just 6 more days left to figure that out 
While he’s not in game, might be able to ‘defeat’ him but you wont be able to ‘kill’ him if they add him in. Exiles takes place vaguely before Age of Conan in the lore.
There’s no real vagaries in when Conan Exiles is set. Joel himself said it’s set between Xuthal of the Dusk (or, The Slithering Shadow) and A Witch Shall Be Born. Well before Conan went and strangled Numedides.
Since the absolute end of the game is already written, I’ll have to say…
Spoilers.
(It is written though, that part is true. Research at your own peril.)
Having listened to what all the current NPCs say about him I would be surprised if he wasn’t the end boss
It’s no question that he’s behind the people being rounded up and stranded in the exile lands; but given the time frame the game takes place in the idea that he’d be any sort of physical presence in the game, let alone something we’d fight, leads only to a situation where’d the player would at best just hinder him a little and he’d scamper off.
Edit: Just for further clarity into it, Thoth-Amon by all accounts is pretty much unchallenged in his bid for power between his finding of the Serpent Ring of Set and some time into Conan’s rule when he lost the ring and was subsequently forced into servitude to Ascalante. The game takes place at a time where he’s still gaining in power.