This article says that peer review is unpaid - it isn't. If you are an academic or work in industrial research, it's part of your paid job to do peer review as part of the community. You are paid for it - it's part of your job and it's normal to do it during work hours.
It is unpaid. A lot of reviewing for conferences is actually being done by Ph.D. students. They have to do it, whether they are on a stipend or not. Equally, a lot of reviewing is being done by (in some cases, former) academics whose current occupation does not provide resources (time) for doing peer review.
This is like saying that undergraduate students are unpaid for doing their homework.
It's part of their education to learn how to take part in the community by doing things like reviewing papers.