It's the truth. If you look at countries like Sweden and Denmark where salaries and tax returns are public information, most salaries are all the same.
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.