I'm pretty sure that's correct Boris borrowing shares against your Margin Account is the only way I believe… low interest loan against the borrowed shares is how it would go. Not valid with a Cash Account.
You are correct Borris, shorts cannot borrow from cash accounts.
Per the SEC website:
"Brokerage firms typically lend stock to customers who engage in short sales, using the firm’s own inventory, the margin account of another of the firm’s customers, or another lender."