That isn't actually true. You *will* get sick even with positional tracking, as many people found out when the DK2 Rift was released. Just look in this thread, for example:
https://www.reddit.com/r/oculu... Positional tracking enhances immersion and potentially presence, but it is not really a fix for motion sickness. Unfortunately many people don't understand this.
The problem is deeper - you are correct that the sensory mismatch between what you see and what your sense of balance (inner ear) and proprioception (nerve endings in your muscles relaying the position of your limbs) are telling you is what causes the problem. However, that is not really tied to the positional tracking. It is fairly easy to demonstrate - many people get sick even with full 6DOF tracking using a very expensive big bucks tracking system, walking around in a CAVE, not using an HMD at all (CAVEs are usually far less motion sickness inducing than HMDs).
Most of the nausea problems are caused by poor application design - sudden accelerations are bad, because you don't expect them (it is akin to someone pulling the rug from under you!), motions not initiated by the user are bad (again, unexpected movement!), inappropriate navigation schemes - strafing, head bobbing, "aiming with your head" (not being able to look and change direction of movement independently - as in all FPS games that use mouselook), etc. All these things cause motion sickness. No amount of tracking wizardry is going to help you there unless the design of the application is fixed - and these problems are unfortunately in almost every single demo that was released for the Rift so far, despite there being 30+ years of published research on VR available.
Then there are problems that are often ascribed to motion sickness, but are not really - headaches, dizziness, eye strain. Those are often caused by a poorly adjusted HMD. This is where Rift suffers a lot, because unless you have perfect vision and your eyes are spaced exactly the same as the Rift lenses, you will get eye strain and headache after a while due to a blurry, out of focus image. This is why commercial HMDs have both dioptric adjustment (the two pairs of replaceable lenses really aren't a solution) and interpupillar distance adjustment (the lenses or even displays themselves can be moved closer or farther apart). Another issue with the Rift-like HMDs is with scenes where the textures and jaggy, not antialiased lines cause visible "beating" (moire) against the raster of the relatively low-res display, provoking a lot of visual discomfort - this was really bad in the DK1, DK2 reduced it a bit thanks to the higher resolution and pentile display. That's why dark scenes work best with Rift, because the dark pixel raster is not that visible.