No one says substrate is aerobic or anaerobic, it's a medium for bacteria to attach, congregate, just like bacteria on our hands will attach and multiply. That's why we need to wash them when possible. There's conditions in substrate, depending on fluid flow, type substrate,type fluid,etc.... which ORP determines which type of bacteria will thrive.
ORP reading will not tell much about the DO content in biological active water and the pathway used by bacteria. ORP reading is the result of many reactions in the water column after all reactions have taken place. The ORP condition in the water column will not determine the pathway followed by different types of bacteria, attached to a substrate or being part of microbial mat, although the one will influence the other.