The Muʿtazilī intermediate-station argument holds that the major sinner cannot be classified as an obedient or complete believer, yet does not fit their category of unbeliever. They therefore assign a station between the two. This is a doctrinal classification, not merely another name for “a sinful Muslim.”
The Sunni response distinguishes lack of perfect faith or obedience from complete absence of faith. A person's serious failure to obey does not on its own prove the annihilation of the underlying assent of faith.