Some Muʿtazilīs restrict the possibility of punishing a minor sin to cases where major sins have also been committed. They treat the promise of expiation to those avoiding major sins as decisive against punishment of minor sins in that case.
The reply distinguishes a promised outcome under specified conditions from an intrinsic impossibility of punishment for every lesser offense.
Avoidance of major sins, commission of a minor sin, repentance, and forgiveness are different variables. The relevant promises must be understood without treating one verse as a complete exhaustive rule for every conceivable situation.