I think what is true is that if one has big issues and one is very stressed, it means that one cannot focus on writing. That, however, is true for everything - not just writing.

I might be wrong, but I always had the impression that people who couldn't find something to write about just didn't have any thoughts or any knowledge basee.

I once ran into a writer (non-fiction, commercial), and she wanted to know how I wrote fiction so easily. She wanted to write crime novels but didn't know how.

I asked her if she had ever read a non-fiction crime book, and she said no. I said, "There's your problem. One cannot write something if one knows nothing about it."