If that were the case, there wouldn't be 53 pages on this thread--and dozens or even scores of other threads on this and other boards--discussing the rule.
At the moment, I'm reading some essays written by The Alexandrian on disassociated rules and why're they're bad, and I think that this is one of them. "no metal armor" is a rule--which may be a rule or it may be a taboo--but no in-game reason for it*, no in-game penalties for breaking it (whether it's a rule or a taboo), and no mechanical penalties for breaking it (ditto).** Everything associated with it is purely homebrew.
*You may say it's because druids are associated with nature, but there are Mountain and Underdark Land Druids, and both of those terrains are well-known for containing natural metal. And while there is currently no official Circle of Ore, I did a quick search and there's several homebrew versions which means people do think about it.
**Again, if you're charmed or dominated--or even just tricked--into wearing metal armor, what happens? Seems like a good way for an evil person to get rid of a meddling druid: offer them a present of magical non-metal Medium armor ("we offer this to you in a token of peace and friendship, made by our greatest smiths and tailored specifically for your needs") but it's actually metal armor with an illusion hiding that fact. Voila!