Being deeply skilled in a domain means understanding its specific terminology and the dominant modeling approaches used in that field.
Explaining things simply, on the other hand, is about deciding what to cut out and what to reinforce when communicating with others or with the public, within a domain you're already skilled in. So they're similar, but I think they're different areas of competence.
For example, when writing, you usually model your readership. And in the process of adjusting your explanation to fit that audience, you inevitably lose some information in areas you're deeply familiar with, or you add things like analogies to compensate