If this is so much of a wildcat territory then why did Exxon,...

If this is so much of a wildcat territory then why did Exxon, the largest O&G company in the world at that time spend $30m in the 1990s? The 1990s was a much more risky time during which the prices of oil and gas were lower than they are now.
In the 1990s there was also optimism that many new oil and gas deposits would be found. This optimism has faded away; for example 2017 was year during which the fewest barrels of oil were discovered since 1942.
Invictus is the first mover in a sizeable country which has many more potential deposits that may be found. If government and management both agree that they want to expand Invictus laterally, then this is a certain option, especially because relations between management and the Ramaphosa government are very good.