It would probably test bad if you were to put a load on it. Its probably a 700-800CCA but the car likely only takes 300cca or so to actually start up. Also the fact that it is under the back seat helps a ton, its perfectly clean and sees more consistent temperatures that way, batteries being out of the engine compartment helps battery life a fair amount.

I work in the battery business and its not uncommon to see remote mount batteries be 6-8 years old sometimes more. How far and how often the car is driven all play into battery life.

The oldest stuff I see usually is in old tractors. Where someone thought that little 30HP diesel needed 800CCA when it really needs like 125CCA and one compression stroke to start, the battery can test bad per its rated CCA for ages before it actually won't start the tractor lol.