Interestingly Force Majeure is not a legal concept in SA law although it is recognised as a clause in contracts. And you can't just declare it - it really has to be about events beyond your control. There is a little test:
Could you control the event or circumstances?
Did you examine the political, social, economic, healthcare, climatic, geological circumstances prevailing at the time of contracting which may have impacted on your ability of performing your obligations?
What actions did you take in an attempt to avoid or overcome the event or circumstances?
Are you not in some way or at all responsible for the event occurring?
So this is really about a truck driver strike and maybe Shell has no part in those discussions. Unless Shell has its own fleet of trucks. Maybe Shell did settle - and their trucks are being targetted because any truck on the roads is a target.
But the thing that I'd think about is the second test - the whole socio-economic environment. And throw politics in too. TIA as they say. And Shell has a long history of fuelling revolution in SA - Throw well, throw Shell was a slogan from the pre-universal franchise days.