Official Word from store owner...take that as you like.
Complete NEW codex - Yes, this is not a supplement.
New Fire Warrior Models - None that I have heard of
I haven't heard ANYWHERE that it is a supplement. At the very beginning we thought it might be, since our stuff was so new, but that suspicion was apparently wrong.
Currently, no new Fire Warrior models have been seen. There ARE new Stealth Suits, a new box for Command Suits, a new unit (Vespids), new Ethereal Models, and two new characters. Plus the Piranha and Sky Ray look to be in the new codex, and not just a Forge World thing now.
...That should catch you up on the basics. There are supposedly (confirmed?) new rules for Markerlights, new options for Broadsides and Crisis suits, a new Heavy Support option (sniper drone team), a new weapon or two for the Commander, and Kroot Oxen and Kroot Hounds are now an Upgrade for Kroot Carnivore squads and not a seperate entry in the FOC.