Everyone except the people uncritically repeating this political talking point agrees that a 90% marginal rate is very far on the right side of the Laffner curve.
Even if it is, and even if your point is true, that's not what the GP said. They said taxing rich people a higher percentage than poor people is "more efficient" (whatever that means in this context) and a "[more] fair system," and immediately followed it up with the 90% anachronism.