Although Luke may have been able to drive his truck and perform peritoneal dialysis on the road, he was not a candidate due to previous abdominal surgeries. The couple sat down with their social worker and found out that Luke had enough work credits to be eligible for Social Security Disability benefits. Credits are earned by working and paying Social Security taxes. He could apply for Social Security Disability Insurance to provide some income until he and Diane could plan for their future. You can receive SSI even if you don't qualify for Social Security.
Disability is determined in the same way for both programs.
Both programs are managed by the Social Security Administration. It is your ability to work that counts.