I'm confused. What does the throttle have to do with it? All tractors have a throttle control to control the engine RPMs, but it is a totally separate function of the HST's groundspeed control (treadle, go-pedal, whatever) or the cruise that works with the HST groundspeed control.
Some newer models will automatically throttle up when you actuate the HST's go-pedals, but that is just to ensure proper flow of fluid because many HST users don't run high enough RPMs to properly feed the beast and that is just a newer "fix" some manufacturers are implementing. As far as I know the throttle control and the HST controls are still separate functions though.
Not disputing anything you said, just seeking clarification.