The model assumes that the 1-month T-Bills are used as collateral for the put options so no cash is needed. Of course in practice the T-Bills do not mature on the same day as the options so there might be some need for additional cash. That should however be rather low since I am selling one month put options and they are almost never more than 5% out of the money at maturity. My guess is that a 5% to 10% cash position plus the rest in T-Bills should suffice in practice but that is something one needs to check.