Listening to the radio discussion between the two, Zille seemed to say the DA had submitted the advert via an agency according to a set of specific, prepaid slots which were paid for. The day before the ad was due to flight, the SABC had emailed the agency just before midnight, to change the ad otherwise it would be refused. The prepaid time slot came and went and there was no flighting. The second slot was for the 9pm programme last night.
The SABC response seemed defensive and argumentative but I could not understand what the guy was actually saying. Reading this article hasn't really cleared up much, except maybe the SABC guy admitted he hadn't really been listening to Zille before he started arguing.
That's my take on it but happy to get more clarity from either side.