Wouldn’t this ‘negative cash’ be a regular financial instrument like a stock short, reverse mortgage, credit default swap, etc., which they actually can use, precisely because nothing is created by this process? It is just less liquid than hard cash, which can affect the perceived trade-able value (but that too is already factored in as expected interest).
Yes, a loan creates both a credit and a debit, and they offset exactly. In that sense, nothing is created.
But the credit spends just like cash. The debit, on the other hand, does not spend like negative cash. So in the sense of the supply of money in circulation, bank loans create money.