The interest reserve does accrue interest on funds drawn from it. Or put another way, interest is charge on interest accrued. You can see this formula in row 30 of the Dev-Interest Calc tab.
Follow up question: How do I reduce the interest reserve number by the amount of available operating cash flow that could be used to pay it before full stabilization and a take-out loan is arranged? This causes the interest reserve to be too high. Am i missing something?