That is almost certainly the case. There isn't much room between the back of the lens and the sensor with R (probably almost none with some lenses). Also, this is a cinema camera and most users are going to be looking at parfocal lenses, so not much real interest in R glass. Lastly, the ultra fast wide angle advantage of the R mount does not relate to a significant percentage of cinema work. In any case, running the rapids is more likely to be shot with a disposable body than a $16k studio camera.