I imagine it is slightly out of context. He’s claiming they should uphold it because it’s the right thing to do, even if the constitutionality of it is a bit... hazy. Not like they haven’t upheld terrible laws before. Remember Dred-Scott?
Dred Scott upheld the constitutionality of the Fugitive Slave Act, which was not only constitutional (in general) but was specifically authorized by Article IV, section 2 :..."No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due"
How on Earth could a court declare it unconstitutional?