It is literally a parenthetical aside, and it's not an exception. If it was an exception, it would have been written as "druids are not proficient in metal armors." Instead, they wrote it as a choice the druid makes.
"If you feel strongly about your druid breaking the taboo and donning metal, talk to your DM. [...] As long as you abide by your character’s proficiencies, you’re not going to break anything in the game system, but you might undermine the story and the world being created in your campaign."
They didn't say "druids aren't proficient in metal armor." They said "talk to your DM; it's not going to hurt anything if your druid wears metal, but it might not be appropriate to the campaign's theme and flavor." They wouldn't say that if it were about breaking a rule.
Plus, every other one of Crawford's tweets on druids in metal armor refers to it as a taboo.
I'm not pretending it's not really a rule because I don't like it. I'm claiming it's not really a rule because it's specifically a cultural taboo with no mechanical or social consequences for breaking it. (Seriously, I asked you what the penalties are. Well?)