Of course it is. Under actual rules druids do not wear metal armour and there is a limited amount of non-metal armours. That's the RAW, anything else is houserules.
So, you are completely abandoning your previous position that non-metal armors are available and fully committing to the fact that they don't exist, and putting them in is a houserule that buffs the druids.
A person could get whiplash trying to follow your arguments.
I am not discussing 'why' with you, as we both know you're not gonna get it. But it is a rule, and rule which the designers put there intentionally, knowing what it means.
I always love how think that fact that I disagree with them means that I don't understand them. That if I was somehow smarter or better I would agree with them. Because they can't possibly be wrong in anyway.
I think the designers did understand the rule they put in. That rule, at its most powerful and strictest reading, prevents a druid from wearing metal medium armor. Nothing prevents them from wearing non-metal medium armor. And anyone with a passing familiarity with Dungeons and Dragons history, is aware that that is a thing that has existed for decades.
Note, a wooden shield is mechanically identical to a metal shield. Note, a metal staff is mechanically identical to a wooden staff. A stone maul is mechanically identical to a steel maul. So, why would half-plate made out of bone or shell be different than half-plate made out of metal? You say it doesn't exist though, and that creating it is an unfair buff for druids, but again, the designers specifically left that door opened and highlighted, and they aren't stupid enough to have not seen that.
Sure. They totally intentionally put rules in the game that they assumed no one to follow and then instead of balancing the game based on the actual rules, they balanced it on potential houserules that people might come up with!
Or tradition in the game dating back to the creation of the game. Since, as Max points out, non-metal armors have been a thing since 1e.
So, let me ask directly. Do you think Mearls, Crawford and the others had no concept that a druid might wear half-plate made out of something like Ironwood? Do you honestly believe the thought never crossed their mind?