I think they've probably just misread the question. Other threads on the Powerbox forum seem to suggest that their preferred installation method, when using an SRS Powerbox, is to install a iGyro (not iGyro3e) between the RX and the Powerbox using it's S-Bus inputs and outputs. You can't do this with a Cortex (or an iGyro3e) as it doesn't have the S-Bus output.

The question actually asked on the forum was whether you can install the Cortex between the SRS Powerbox and the servos and the answer is, in theory, yes you can; however I haven't tested this combination myself.