I strongly disagree. A person who loves to read pop sci physics will be much better at writing clear sentences about physics than a physicist, but the physicist will for sure have a better understanding of physics.
The world is very complex and almost nothing in it can accurately be summarised in a short piece of writing. Being able to making decent summaries is useful but it doesn't show mastery of the subject nor does being unable to do so show lack of mastery. Rather many masters feels that the summaries are woefully inaccurate and therefore be unable to write them while many amateurs will make very clear summaries of fields they understand nothing about.
The world is very complex and almost nothing in it can accurately be summarised in a short piece of writing. Being able to making decent summaries is useful but it doesn't show mastery of the subject nor does being unable to do so show lack of mastery. Rather many masters feels that the summaries are woefully inaccurate and therefore be unable to write them while many amateurs will make very clear summaries of fields they understand nothing about.