What is it about “the Congress has to approve the deal for it to go into effect” that you don’t understand?
The part where the constitution requires 2/3 of the Senate to approve, but Fast track only requires a majority vote, and then not to approve, but to disapprove.
And the part where Congress can't amend or filibuster.
And the part where the bill is kept secret and pubic doesn't get to debate it.