No fire NOT a firebox.
NOT an accurate or legal description.
That’s what the manufacturer calls it. It’s called that all over the internet.
From webster:
Definition of firebox
“1 : a chamber (as of a furnace or steam boiler) that contains a fire.”
example:
“Each 600-pound steel pit includes a double-walled firebox , 42-inch cook chamber with a water pan shelf and cooking grate, and a removable smokestack that reaches more than six feet high.— Andy Wang, Robb Report , 4 Aug. 2021”
We’re gonna use the name that the manufacturer uses. 
If it looks like a fire it’s called a firebox.