I think this is very closely related to the unity-gain of operational amplifiers.
That is, I think the SMPS you're talking about uses op-amps to tame their system.
Op-amps has a bandwidth that is related to their unity-gain frequency and closed loop amplification which can be seen in the graph below.
Yes, that's right, real op-amps are designed to have one dominant pole, and you as an engineer can choose where on the slope you want your op-amp to operate in.
The equation for the bandwidth is the following:
As usual there is always a trade-off, either you have high bandwidth and low amplification, or you have low bandwidth and high amplification. You can however cascade amplifiers to increase the bandwidth, or use better op-amps, both of which costs more money. It's all about trade-offs upon trade-offs.
In your case they are talking about "peak current mode" which is something that should be dealt with fast. Fast means high frequency, high frequency means high bandwidth, high bandwidth means low amplification. And this is why there is lower gain for peak current mode control.