FIXED!

You have to use a number lower than 15% to get the throttle cut to actually work. The lower the value, the more it closes the throttle.

setting >15% - Throttle CUT is actually THROTTLE INCREASE, not throttle cut. What were they thinking with this unexplained function?
setting =15% - no action
setting <15% - Throttle cut actually works. Lower numbers increase throw. 0% is the max cut-off.

The default for Throttle Cut is 17%, which looked like it does not move anything. Looking more carefully, with my Hitec RX and servo, which I set up as a test, the servo nudges slightly in the opening direction at 17%. 17% might be the neutral setting with a Futaba RX and servo. Since it didn't do anything much at 17%, I increased the setting to a higher value, like I would on a 9C. The amount of throw increases as you increase the setting to the 50% max value. This gives the appearance that it is working backwards. From my tests, it didn't make a difference if the throttle servo was reversed or not.

Just use a setting below 15% if you want Throttle Cut to actually CUT the throttle! 9% worked in my application.

In hindsight, using 0% and 50% make some sense, just very different philosophy than the 9C. If you think about the end point at the bottom being 0%, then setting it at 0% brings the servo to its endpoint. Setting it at 50% brings it up to 50% up from the endpoint. I'm not sure if this was their intent, but it's how I've rationalized it!