CFAI EOC Reading 51 Qn 27 (Pg 217)

This question relates to the credit analysis material on fixed income. In the CFAI EOC Reading 51, Qn 27 (Pg 217), the question asks us to calculate the debt payback period, which has the formula of total debt / discretionary cash flow. However, the solution uses the value for total liabilities as the value for total debt, why is this so? Shouldn’t the total debt value that is used to answer this question be short/current portion of long-term debt plus long term debt??