"If you are a true master, you should be able to explain concepts in a way that even a child can understand. "
This isn't needed to be a master in a field and it isn't necessary unless you're speaking to novices or people outside the field. Sagan, Hawking or Feynman are good examples of this. Einstein was a real aberration, where even some of his papers were written with disarming clarity.
For Trump, I think you're mixing this up with the Dunning Kruger effect, where a person's inability to understand what's going on around them makes them think they have a better understanding than the experts.