We've established that BMW does have a 6MT in its arsenal that can handle more power and torque on the V8s. Which BTW is the same general architecture they've been using for years and adapted to many models.
I'll tell you what I think the true reason for the detune is (and you were right there!): cost.
The V8s use a beefier twin-disk clutch. T̶h̶e̶ ̶i̶6̶s̶ ̶h̶o̶w̶e̶v̶e̶r̶ ̶u̶s̶e̶ ̶a̶ ̶m̶o̶r̶e̶ ̶s̶t̶a̶n̶d̶a̶r̶d̶ ̶s̶i̶n̶g̶l̶e̶ ̶d̶i̶s̶k̶ ̶c̶l̶u̶t̶c̶h̶.̶ edit1: was wrong here, the S55 got a twin-disk
I imagine BMW didn't want to spend the $ necessary to come up with a new/upgraded clutch for the more powerful i6 (especially since they probably don't foresee selling many). Instead it's probably getting the same/similar hardware that's on the F80 hence the identical torque rating.
edit2: nailed it, same clutch as the F8x