I definitely agree that the rule is terribly worded. I think it would be clear and balanced if druids just had light armour proficiency.
Now personally I wouldn't want druids to wear metal armour flavour-wise, and if they could, they would be strongly encouraged to. It basically would become the default that they do wear breast plate or half plate and this just doesn't really fit how I see the druids. Furthermore, druids are already one of the most powerful classes, so they don't really need an AC buff.
As for padded and ring mail etc, the whole armour table is terrible. It has useless trap choice armours that have no mechanical purpose as well as weird historical misconception armours. There is mechanical need for just five different armours:
Light Armour | | | |
Leather/Padded | 12+ Dex mod | | |
Medium Armour | | | |
Chainmail | 14+ dex mod (max2) | | |
Scale/Brigandine | 15+ dex mod (max2) | | Disadvantage |
Heavy Armour | | | |
Half Plate | 17 | Str 13 | Disadvantage |
Full Plate | 18 | Str 15 | Disadvantage |