That's not true, the approval would only be relevant if ONCS were to have to buy it on the open market. If they partner, Merck could provide the unapproved drug for the clinical trial, just as they are conducting their own trials with an unapproved drug.
I'm not sure when Merck's goes to market. BMY should beat them. However it does not have to be approved for ONCS to do a combination trial with it. Though that may be an event that they want to see happen first for improved partnership deal. Its hard to guess, and we are probably wrong at what we guess anyway