You're saying that it stopped being evolution just because the engine finally went from air-cooled to water cooled? That seems like a feature upgrade, which is an evolution; not a new product.
The article you link to calls the 996 the "first truly all-new 911."
The 996 replaced both the head and the handle at the same time - but all 911s have evolved along the same lines,and the 996 just followed that line.
996: air cool -> water cool 991: hydraulic -> electric steering
Even from a physical standpoint the 996 on are positively large compared to 993 and earlier. Isn't a cayman about the same size as pre 90's 911s?