Of course congress could amend the constitution, which I understood, but that takes 2/3rd of the states to support it. They can't make a regular law to overturn the supremes, right?
As the only body capable of updating the text of the law they have ultimate power. That they chose to constrain it was an altruistic act of a sovereign body. But only they (and their predecessor the continental congress) had the power to restrict themselves. The courts can only hold them to the text they themselves wrote.