I actually mused exactly the same thing, that perhaps it was a confused reference to a Merlin with the sort of two-stage/three-speed supercharger used on the 100 series Griffon. The original Merlin engine schemed for the Merlin Spiteful variant was the RM.16SM (as used in the Spitfire PR.Mk.X). Of equal interest, to me at least, was an assertion that a Spiteful with the Merlin would have approximately 8-9% more range than the Griffon powered version.
On a lighter note I found myself having considerable sympathy for the people at RR and Supermarine in particular. After all the work undertaken on developing the Spiteful and the high-altitude Griffon to power it the Air Staff decided in mid-late 1945 that their only interest in the type was as a low altitude attack aircraft, so the whole programme was held up as Supermarine worked to install the ability to carry a 1,000lb bomb under each wing.
I didn't think this picture would work, but it does just about, its from the Spiteful files I was looking at, Griffon 65, versus Merlin 66 versus RM.16SM: