There's confusion on the meaning of "sanctions". There is no trade embargo on Zimbabwe. Any such embargo would have to be tested under WTO rules. There was a GATT-consistent trade embargo against Rhodesia, yes. But the "sanctions" against Zim do not include a trade embargo.
So the argument that Zimbabwe should counter "sanctions" with import substitution is completely irrelevant: Import substitution was a policy meant to address directly the trade embargo Rhodesia faced. As above, there is no such embargo so import substitution does not arise.
In addition to import substitution, Rhodesia was assisted greatly by the sanctions busting of British and South African companies. BP was famously nationalised in Nigeria after its sanctions busting activity in Rhodesia.
