It is not the same as the normal split frequency. IF you understand the math behind FS, Orig=HF+LF. In a normal FS all 3 channels undergo the Apply Image command. In this LAB version, only the L channel is split that way. The A & B channels are not. In an RGB image there is lots of color on the HF layer, it is not all grayscale. The LAB split is asymmetric from two standpoints: The L is split but the A and B are not; There is a full separation between color and texture.
You can perform a normal FS in LAB by splitting all 3 channels but the results are not the same.