Not exactly. You can defend property with normal force, and if that turns deadly you can then be authorized to use deadly force, but the deadly threat does still have to present itself.
As in, he could have walked outside, gun in holster or even in hand at low ready, and said “get the fuck out of here,” or punched or OC sprayed him (of course, this is all if he was actually stealing the car, since he wasn’t this would also be assault, but ykwim), and then if the guy pulls a knife, or blunt instrument like a pipe, or goes for a gun instead of retreating, then you can shoot him.
These laws are all very state specific, as well, but by and large that’s how it works, you can’t just start blasting because “well anyone could have a gun or knife.”
That said, it’s still up to the DA to bring charges and the jury to convict, even though it is a crime I’m sure you can find a case that fits the description where the guy got off, hell OJ got off, but it is still illegal. In this case the DA did bring charges, which indicates to me it’s illegal enough that the DA thinks they can win.