What he did was the definition of racism. They dont need white guy to run it like you said but you also claimed their are no qualified black people in the same sentence. Color of your skin is just a thing but sometimes the color of your skin is a reflection on much deeper truths and facts and that you cant get away from.
I wouldn't take it so literally, but there is causality there. If most of your farmers are white then obviously there are going to be significant problems if you get rid of them all and bring in a bunch of people who haven't done it before, doesn't matter what colour their skin is. Mugabe's mistake (on top of a million others) was not having a contingency plan to replace them. He acts then thinks.
What ended up happening is that the inexperienced black farmers, with their newly re-stolen land, starting hiring the experienced white farmers as managers. Funny eh? In the mean time Zimbabwe currently have a bunch of yellow farmers trying to bridge the deficit. Absolute mess, but so often in Africa when something goes wrong the organisation is not there to compensate.