That's not really because of the tax rates being equal, there are many other reasons. For example, higher education is free, and labor organization is very high. This keeps blue-collar pay closer to white collar, flattening pay structures.
Yes. And I argued that in order to claim that this is what flattens structures rather than the many other factors, you need a control sample that has Scandinavian labor market structure (High tax, free higher education, single payer healthcare) BUT with non-public income. I'm not sure such a country exists.