These are not the same and I sometimes confuse myself, but in short reprojection is compensating for render-to-photon delay (which inherently shows as black edges when turning the head fast), and motion smoothing is creating new and inter-mixed frames (like Sports Mode on the TV). This is all better explained in Alex Vlachos videos and slide show in the OP.
Sometimes you can call both “Motion Smoothing” anyhow, because I believe the 1st one (reprojection) can’t be disabled in any case as it is essential to VR rendering. This didn’t occur to me before why 120Hz or 144Hz is better with the Index but now that the latest SteamVR is working much better, I can see why. With a higher refresh rate, the VR API is “projecting” the same frame more often which in effect is reducing the apparent stroboscopic illusion you get with a lower refresh rate. It is really impressive when you’re comparing the Index at 80Hz and 120Hz when in the first case on violent rolls looking over the wing the scenery horizon, you can see a “ghost gap” between the view where it was and the view where it is displaying now, whereas at 120Hz you barely notice it because it is displaying more often the same frame in between. It has the effect to also reducing juddering because with more of the same frame displaying, the gap between the contrasted line for example is shorter and therefore less noticeable.
In turn, it makes any comment such as “anyhow no game can render at 144Hz” moot, because the main point is not rendering at 144Hz, but to re-project 30Hz or 45Hz frames more often. I never thought about this this way before!