I don't think that's true in every case. When I first started playing VR I would sometimes get a little motion sick. I kept playing and only took breaks when it got bad. Now almost nothing makes me motion sick except when my headset massively bugs out and the world freezes and even then I don't get that sick just a bit uncomfortable.

I've heard of other people purposely training themselves to not be motion sick by playing fairly extreme games or in one case a developer who built a world that would repeatedly drop him from a height.

I don't doubt that it's possible to have the opposite outcome, it probably depends on the person and the process they're going through. I just don't think it's a general rule.