The problem with this argument is that we know what happened when humans met trusting animals: we ate them to extinction. So probably the horse’s ancestors were pretty fierce.
We ate all of them when we showed up, in the american case c. 10000BC, long before iron weapons.
My point is that if the wild horses we first met were similarly tame, then surely we would have eaten them too. We didn’t. So probably they weren’t.
So that can’t be what stopped us domesticating african animals.