It wasn't just the engine. The 996 was a new design. The 911 has been replaced twice, once in 1998 and once last year. (http://en.wikipedia.org/wiki/Porsche_911)
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?