I think you're partly right, but I also think that locals DO know about the car park, but have various reasons for not using it:
If you live inside the South Circular, you have to get on to that infernal road for the sole purpose of coming back off it into Waldenshaw Rd. That is no fun and takes time.
There is ONE route into the car park but THREE roads out. The approach is uninviting, and then you are faced with out-of-order machines.
Possibly Davids Road would be better changing the direction of its one-way traffic. Turning right into the South Circular from Davids Road is dangerous, and the left turn is very sharp - difficult to avoid crossing the centre of the South Circ into oncoming traffic. A left turn from South Circular into Davids Rd would be easy and safe. But to change the one-way direction there would need to be a new mandatory right turn into Pearcefield Ave for vehicles coming down off Manor Mount.
[This is off topic, but IMHO traffic is a major problem in Forest Hill. The Council has a very conservative 'nothing we can do about it' attitude, which is unacceptable. Other boroughs are canny in discouraging or diverting through traffic out elsewhere, but Lewisham seems happy to provide free roads for drive-thru commuters at the expense of residents.]