Technically, no, though their belief has often been thought to be so.
A more accurate term would be that they believe in “soul death”. They reject the immortality of the soul and say that a soul can die, not sleep. They teach that the soul is a combination of “the breath of life” and your physical body. Once the “breath of life” departs from your body, the soul no longer exists. Once a person physically dies, they aren’t sleeping, but rather have ceased to exist. So their soul has technically died.
But Jesus Christ taught in Matthew 10:28 to not fear those that can kill the body, but not the soul (showing a dichotomy). Instead fear Him who is able to cast both body and soul into Hell. If the Adventist teaching on the soul were correct, then man could kill the soul by killing the body.
The Bible teaches that the soul/spirit possess a conscious awareness to some degree that is distinct from the body. Paul told the churches at Corinth that he’d rather depart his body and be with the Lord (2 Corinthians 5:8), but knows for their sake it was better that he remain “at home” in the body. He implies that one can be aware while away from their body. It’s clearly seen in Luke 16:19-31 that people who have physically died are still consciously aware in either the lower compartment of Abraham’s Bosom (Hades) or the upper compartment (Paradise) as they await the resurrection from the dead to be reunited with their glorified bodies. It’s also clear from scripture that the spiritual realm is real (Matthew 17:18) and spirits do not have physical bodies (Luke 24:39) yet they are consciously aware.
Furthermore, when Jesus Christ died physically, He did not cease to exist for 3 days to then have His character/personality infused back into a new body made of new particles. He was the fullness of the Godhead bodily (Colossians 2:9) and upholds the universe by the word of His power (Hebrews 1:3). Since he was fully human, if the Adventist view were true, it would require that Jesus ceased to exist for 3 days.