No. of Recommendations: 3
Presumably they heard something in the speech that tagged Mr Zuma as being part of their own over-broad mental category already containing Mr Trump, and saw slighting of one as slighting of all.
They also probably, and perhaps correctly, saw it as an allegorical commentary on Trump as well.
The popular television show M*A*S*H was set during the Korean War, and only depicted the Korean War. They never mentioned the Vietnam War once, since it would have been anachronistic to the setting of the show. But it was still very much about Vietnam as well.
Whether the same is true of the speech about Zuma will depend on the particulars of the speech, of course. But it's entirely possible that their reaction was more rational than the OP is giving them credit for.