You make an excellent point. This takes one of the longer standing arguments from NDZ's camp and neutralises it completely: that it's time for a woman president and NDZ is the only viable candidate. If we then ask, who benefits, then Ramaphosa likely does the most. Which would further indicate that we aren't waiting for horse trading to occur as Grootes writes, but that horse trading has already happened: "Step into the breach, Sisulu, to effectively neutralise NDZ's campaign, and you have yourself a Deputy Presidency, if I become President." 

To be honest, I first suspected that Sisulu was a Zupta ruse to split the vote. But the above arguments seem to make more sense, as these developments seem to be in Ramaphosa's favour.