I_sec has considerably high harmonic content and it can be in DCM too if you are operating below resonnat. There are other modes too that change with frequency, Q and K. I_sec needs some filtering.
We are just filtering here those harmonics to have a somewhat a dc value as I_fb. Then we can inject small ac perturbation to see the impact of ac small signal over the I_fb.
Also as we maybe aware that the PI controller we design either for voltage loop or current loop works better for error in dc quantities hence I_fb needs to be a somewhat filtered as a dc quantity.
Then we get open loop current loop response and we can see there the open loop bandwidth. Based on controller design we can select the inner bandwidth to be 1/5 to 1/10 of the lowest switching frequency. Then the bandwidth of the outer voltage loop is 1/5th to 1/10th of the inner loop. Hence, the current loop response is still faster than the voltage loop response in this case.
This is how I have designed this particular dual close loop on resonant LLC.
I hope I have understand your question and answered satisfactorily.
Please let me know if I am getting it correct and if you have any other questions.