This question was answered privately but I will post the answer here for other people to see.
Storm Injector is not part of RORB and is created and maintained by others. Therefore, I can’t say for sure how Storm injector handles the loss calculations when dealing with indirect impervious fractions. My understanding is that Storm Injector uses the command line version of RORB. The command line version of RORB by itself is unable to handle the direct and indirect impervious fractions functionality that is available in normal RORB.