While it's good you called this out, it is technically correct.
This is because it stems from the idea of a business as a going concern. In that context, the deposits paid do not fall under debts as they are not expected to be paid back. As a going concern, they are prepayments towards G&S purchased, so not debt as such. (Of course if the business was being wound up they would be added to the list of creditors, so debts at that point.)
OTOH, the reason to calculate net debt is to see the coverage of liquid assets (e.g. cash) as true debts need to be repaid. So in this context again they are part of cash that can be paid to cover those debts. (The use of cash for these purposes is legitimate as part of business operations. Deposits do not have to be held until the product/service is supplied.)
You are considering things from a liquidation perspective, but that is not how an operating business is assessed.