Actually the first production Spitfire powered by a Griffon engine was the Mk.XII. It is true to say that the Mk.XIV was the first production Spitfire to carry a two stage supercharged Griffon engine though which boosted the Spitfires power to a staggering 2050hp.
The very first Spitfire to carry the Griffon was actually the Mk.IV but that was a test aircraft so it doesn't count.
Seriously don't worry about it. You can't help being told incorrect information.
I would have made the same mistake I'm sure if it wasn't for the fact that I'm a British aviation enthusiast. If there is one aircraft that a British aviation enthusiast should know about, it is the Spitfire.