How does a random post from a different forum prove anything? The Sage Advice compendium is the closest we get to an official answer*. It's quite clear (bolding mine):
Druid
What happens if a druid wears metal armor?
The druid explodes.
Well, not actually. Druids have a taboo against wearing metal armor and wielding a metal shield. The taboo has been part of the class’s story since the class first appeared in
Eldritch Wizardry (1976) and the original
Player’s Handbook (1978). The idea is that druids prefer to be protected by animal skins, wood, and other natural materials that aren’t the worked metal that is associated with civilization.
Druids don’t lack the ability to wear metal armor. They choose not to wear it. This choice is part of their identity as a mystical order. Think of it in these terms: a vegetarian can eat meat, but chooses not to.
*as far as I'm concerned the DM at the table is the official answer, but that's another topic.