Seems true to me
What they have told you seems to be true to me. If you are only validating your license BUT not Converting to a South African one... U may only be allowed to fly what's on your Canadian to be Validated license.... and to be allowed to fly it solo after validation is completed...that's between you and the owner/company of the light twin...but it might also be a tough decision for them to let you fly away in their Baron alone in the wild of Africa!
And to what I know is that south African CAA may not be able to add a new Type rating on your foreign license, so with that said, there is no possibility of you flying a light twin which you are not rated on... But you will be able to fly the light twin currently in your license once Validated... Other conditions may also apply.... to better to check with the CAA very well.
That is my opinion...some one can add onto this or correct me if necessary.