There seems to be a dearth of responses, which implies confirmation of my suspicion that that this is a bit confused.

There certainly is a Gold benefit of waitlisting for revenue flights, however that only applies to Y for the card holder only. I can't quite see where the "needs to be revenue availability" point would work - since by definition there's a wait list. I am not aware of waitlisting for redemptions - at least not officially. Indeed BA, unlike many airlines, tries to absolutely minimise its standby and waitlisting processes since they get in the way of their complex revenue management algorithms.

There is also a published Gold benefit for opening up redemptions for double Avios, again in Y, and yes this does require revenue availability.

So the agent seems to have portrayed a mixture of these two published benefits.

On the other hand there is an unpublished Gold benefit whereby if there is a significant change to a redemption booking by BA, then BA will consider sympathetically changes to the booking, including looking at revenue buckets. However the agent would not have direct power to approve this, it has to go "up the line" for sign off. Due to the big change to the South Africa flight timings this is what I think (stress: think) has happened. Certainly something similar has happened to me.

In which case you will probably hear back within a week or so.

If you really have been waitlisted then obviously that will require the ebb and flow of bookings to wash over the process. Travel agent bookings are the real variable on this, and about 2 weeks before the flight BA will try to try to get them firmed up then, but obviously it only takes one extended family to change their plans for the waitlist to get moving.