Land Druid and it's AC problems

[Knaight]

So how are they proficient in armour they cannot wear?

They have the physical conditioning needed to use it, and are used to comparable armor made of other materials. It's hardly something that needs extensive explanation, particularly given the rest of the system's propensity for new powers out of nowhere. RAW makes it clear that druids won't be using metal armor, while also making it clear that it's a trivial detail easily changed without worrying about too much.