As far as I remember, I heard about it from a pretty reliable source.
The problem was actually the torque on VW's V10 Diesel but also on high power versions of the Cayenne Turbo. And they needed an automatic tranny to be able to work with 4WD systems too.
Porsche usually uses the Mercedes tranny on their 996 models (996 Turbo and 996 facelift). On the Cayenne Turbo, this wasn't possible. As far as I remember, the Mercedes tranny wasn't ready for WD and for more than 600 Nm torque. If you have different information, tell me.