Yes it’s deliberately worded to be vague it could really mean either with no source, I did some research and most of what I found was multiple books are being published by people, with that same 1/10 being repeated, didn’t look into publishing rates or anything yet though.

Keep in mind, that’s just the people famous enough to have English Wikipedia articles.

There’s no way 1 in 10 people in Iceland are full-time professional authors publishing a book every year.

In the US, most authors I personally know write for fun and have a novel or few on something like wattpad and they’ve made exactly $0 on it.