“everyone is subject to the same rates for the same income”
Yes. That’s the point. People with the same income have the same rules applied to them. People with other incomes get other rules. Different rules fir different classes. That is legal inequality.
“All married couples pay the same rate for income in each bracket”
Yes, but different brackets pay different rates. Different rules for different types of people. What are you not getting? Even SCOTUS didn’t deny it. All they said was it’s okay because the want if revenue trumps it.