12 step groups are not "cults"--unless it's a splinter group following an absolute authoritarian leader.
Other than such splinter groups, which are relatively rare, 12 step groups don't fit the criteria that defines a cult.
Psychiatrist Robert Jay Lifton, who wrote the definitive book about thought reform (often called "brainwashing") also wrote a paper about cult formation. Lifton defined a cult as having the following three characteristics:
1. A charismatic leader, who increasingly becomes an object of worship as the general principles that may have originally sustained the group lose power.
2. A process [is in use] call[ed] coercive persuasion or thought reform.
3. Economic, sexual, and other exploitation of group members by the leader and the ruling coterie.
The single most definitive and salient feature of cults is the charismatic leader, who becomes an object of devotion, and has little if any meaningful accountability.