Let's put it this way:
1. It is a relatively low impedance compared with the output of the push pull stage(~ M ohm).
2. Just imaging a normal voltage regulator design(without any large capacitor at the output), the output is a high impedance node. And the worst case is when the current is extremely low, by which I mean it is the worst case for stability. This leaves two high impedance nodes, which is potentially unstable. When the current increases, the equivalent load resistance decreases and the output node is pushed to higher frequency, so the phase margin improves.
3. With the + terminal connected to output of regulator. As you calculated, even in 1uA conditions, the impedance is still ten times low than the output node of push-pull stage. It helps for the stability.