As I stated earlier, he still had to agree to the defense unless there is a court order declaring him mentally incompetent.
I think it's a legitimate question why he would be competent to stand trial, but not competent to be executed. I'm not interested enough in the issue to read the opinion, though. That said, it's not a very high intellectual bar to say okay if your lawyers ask if they can request that the state not kill you. Even a blanket statement such as, "I don't want to die," totally to be expected from even the most feeble-minded among us, would likely constitute sufficient consent.