Does the absence of punishment for drinking alcohol in man-made laws constitute an act of making forbidden things permissible (istihlal), even if it is not stated that drinking alcohol is permissible (halal)?

Not punishing a drunkard is not considered an act of deeming alcohol permissible unless it is accompanied by the belief that Allah has made it permissible. Deeming something permissible (istihlal) is the belief that what Allah has forbidden is in fact permissible, and this is disbelief (kufr) because it is a denial of the Sacred Law. It can also be disbelief if it is a defiant disobedience, like the act of Iblis. As for a mere sin, it is not disbelief.