Flight Idle results in a prop blade position safe for flight and still noticeable forward thrust, while in Ground Idle, even some mild reverse thrust may be delivered (DISC and slightly above).
True, but not quite the complete story. In fact, the propeller blade angle for Flight Idle (FI) is designed to produce zero thrust at Vsl, (the power off stalling speed, in landing configuration). It produces slightly negative thrust (i.e. 'drag') at Approach Speed (1.3 Vsl), idle thrust at Touch Down (TD), more or less, and positive thrust at zero speed.
On the other hand, discing produces negative thrust (drag) at TD and zero thrust at Zero IAS, approximately.
Like all reversing propellors, or jet/turbofan reversers, the effectiveness of reverse decays as a square function as the IAS decays. So, the message is: get on reverse early where stopping distance is critical, be it jet or turbo-prop.
This is the great advantage of the turbo-prop: with 'beta' control (incremental control of propeller blade angle with Power Levers) the pilot has a tool to precisely control thrust/drag during the flare, touch down and rollout. But he needs to understand the underlying principles!