A person may sin because of appetite or anger while still admitting that the act is forbidden. This differs from istiḥlāl: regarding an unquestionably prohibited matter as lawful through rejection of its established ruling. Doing a forbidden act does not logically entail affirming its permissibility.
Someone who drinks an intoxicant while saying, “I know it is prohibited and I was wrong,” is not in the same theological position as someone who rejects an established definitive prohibition. Similar outward acts do not necessarily establish identical convictions.