Actually, Walter Martin never included Seventh Day Adventism as a cult. Although he considered some of their teachings as wrong and unbiblical and openly criticized teachings such as mandatory sabbath observance, the investigative judgment, etc., on the central doctrines of the faith (the Trinity, the virgin birth of Christ, the resurrection, etc.) he considered them to be right on.
He wrote a whole book on the SDA church called “The Truth about Seventh Day Adventism” which marked his divergence from those who call them a cult.
I worked for Martin’s organization, Christian Research Institute, for four years when I first got out of college in 1966. I knew him well.
Thank you for correcting my misinformation. I was working from a rather faulty memory.