Faolyn
(she/her)
And that's a specifically magical effect which actually makes a lot of sense in the context of the class and pact. You are creating a special magic weapon. You're not picking up a random weapon and gaining an ability with it; you are either creating a weapon out of thin air or magically transmuting an existing weapon into a pact weapon. It's actually no different than something like elven chain, where you can wear it with no penalty even if you aren't proficient in Medium armor.Blade pact makes you proficient with your pact weapon. Just that one individual weapon, not all weapons of that type.
But there's nothing in the writeup for druids that says that they are magically granted proficiency in anything, or magically barred from using their proficiency based on metal. It's just a "will not" note.
Edit: It should also be noted that this is one specific pact for the class. There's nothing that forces you to take that pact. However, all druids, no matter their archetype, no matter their origins, no matter their philosophy, are forced to abide by "will not wear metal armor."