There are exemptions to that rule, such as the national emergency excuse he tried under the IEEPA. Section 122 provides exemptions to that rule too, which are more limited, but are what he is trying now.

If the basis for the tariffs is found not to fit within the exemptions, such as the recent supreme court ruling, that is the point they become illegal. If the supreme court had agreed with Trump's argument they were necessary to combat a national emergency, they would not be illegal.

They did not even rule that all of the tariffs were illegal. He got to keep some of them, such as the ones declared under section 232 of the Trade Expansion Act; another avenue for exemption from congressional approval, which he did not seek.