I don't know what actual discrete refresh modes are supported by ProMotion, but I can describe the behaviors I saw on the latest stable build and the latest beta. On the stable build, scrolling with your fingers on the touchpad looked like it was above 60hz, but with some strange inconsistent frame pacing or frame dropping (which could be explained by the screen aggressively changing back to 60 if it decided that the motion was too slow.) And when releasing fingers from the trackpad and letting the inertia take over, it hardlocked to 60 every time.
On the beta, the interial scrolling acts a lot more like regular scrolling. It's a lot smoother on average, but doesn't look or feel like a 'clean' 120. To my eye it looks like it's still switching back to 60hz way too fast and way too often. But occasionally, maybe 1 in 5 times or so, I do a scroll motion and it just looks perfectly smooth, which never happened to me on the release build.
It could be just placebo and I'm going insane, and I'm too lazy to build a tool to try and measure the changes in refresh rate, but this is what the experience has been like for me at least.