Actually, there's a "mode" you need to go into which puts the throttle into Taxi/Reverse. In the real aircraft, this is aft of the Flight Idle position.
What you need to do is assign a button on your joystick/yoke to turn on "reverse" in the X-Plane preferences, or a keyboard shortcut if you aren't using one of those.
What I do is press the reverse key I've assigned when the throttle is in Flight Idle. At that point when I push the throttle back up its as if (again the real plane) the handle is aft of flight idle and you'll see you get proper taxi speeds. If you pull back far enough you'll see you go into "reverse" which is not recommended during taxi.
Full Reverse can be/is used on landing.
When you push the throttle up a bit higher than taxi it reverts back to flight idle so you may have to press reverse again if you go there by mistake during taxi.
The coolest part is that when you're in reverse/taxi mode you see the throttle graphic on the screen so you know you're there.
Once again - this aircraft is incredible!