There is no flavor here. There is only a mechanic that lacks an explanation.
As many others have already said "will not" is not a mechanic. It's a flavor. "Cannot" is a mechanic. "Will not" isn't. As I pointed out before (and you ignored), other classes show their restrictions by indicating what they
can use. Not what they can't use, but what they can use. Wizards aren't proficient in Simple Weapons, except for clubs, maces, javelins, and shortbows. Instead, they're only proficient in a handful of weapons.
It says, the Druid armor proficiencies grant light armor and medium armor. However, Druids "will not wear" metal armor, is added parenthetically because it is a special restriction. Other classes dont have parenthetical special restrictions. The Druid appears to have the only restriction that relates to the material that the armor is made out of.
Because the metal itself is the issue, it would disinclude leather armor that was "studded" with metal.
In order to become proficient with an item, one must learn how to use it. Refusing to use it, prevents training the proficiency.
There is not a single other instance of that being the case anywhere in D&D. Fighters are not not-proficient in a Simple or Martial weapon they don't use.
And here's a question: You start out as a fighter, with "proficiency in metal armor." You spend a few levels wearing a metal breastplate, a metal helmet, a metal shield. You multiclass to druid. What happens then? You clearly didn't refuse to use metal during this time.
A mountain dwarf is proficient in light and medium armor. Do you, an adult mountain dwarf druid, suddenly forget how to use metal armor, even though you were taught how to use it long before you became a druid?
"Refusing to use it" doesn't indicate a lack of training. A person can have a driver's license but refuse to drive. A person can make a great steak but refuse to eat it. A fighter can be proficient in Martial Weapons but refuse to use any of them.
The answer is very simple: Druids are fully proficient in Light and Medium armor. They make a choice to not wear armor made of metal because they view it as taboo. And just like with clerics and warlocks, there is no official ruling on what happens if they break the taboo.
The end.
In any case, the special restriction against metal is explicitly in the description of what the Druid class grants proficiency with.
No, it's a badly-placed note, because they know a lot of people weren't looking at the lore. Heck, it was probably a holdover from the playtests back when this was "D&D Next" and the playtest packets didn't have any lore, only the briefest of descriptions.
If you don't want druids to wear armor in your home game, that's fine.