Author here. Agree, and I wrote in that section "Absolute accuracy is hard to compare across markets on one platform, and across platforms, because different forecasting questions have different difficulties. I addressed this by tracking similar markets on a single platform over time."

Even doing this, it's not apples-to-apples. One thing is, in this article, I filter only to "interesting" markets, so that controls for the % that are "easy" as you describe.