Unless, the designer felt it did not need to be a cant, wanted to leave it flexible to cover the variety of lore (sometimes contradictory) across editions and setting and they knew it wasnt gonna break anything if a gm decided for their setting it was fine to go with any proficient - as sage replied.
See, its almost like they chose a more GM ruling centered approach as opposed to a one-rule for all for this non-balance bresking element of mis-matching legacy.
If they wanted to leave it flexible to cover a variety of lore, it shouldn't go into the Class Features, but rather go into the lore sections of the class, like they do for every other class, and then actually add some narrative to said lore. The Sage's response was problematic because it didn't actually address anything except say that there used to be such a limitation back in the original PHB before Forgotten Realms lore was even a thing. Sure, of course a DM can rule whatever the heck they want about anything, and saying something so obvious doesn't answer the concerns with how the line is worded and placed into the Class Features. When a question or concern is raised about a ruling, it's lazy for the Sage Advice to just say "Your DM can just ignore it." The Sage Advice specifically said "If you want to depart from your class's story", but they don't actually provide any story within the class as to why there's a taboo against metal, and ignore the fact that there's a deity in the base 5E pantheon that specifically tell their Druids that it's fine to wear metal, and actually highly encourages it.
That is your interpretation. There is nothing in the rules to indicate that. It has the advantage of not being subject to the Heat Metal spell and certain other traps that key of metal.
Actually there is. It says so right in the description of the armor. "This crude armor consists of thick furs and pelts. It is commonly worn by Barbarian tribes, evil humanoids, and other folk who lack access to the tools and materials needed to create better armor." Studded leather is simply better, as it's also a leather armor and therefore not affected by anything that keys off metal, is a light armor, weighs less, has no dexterity cap, and is stated in the Sage Advice to be a common armor for Druids. There's literally no reason to wear hide unless your DM puts you into a poverty campaign, or your DEX is at a +2 or less AND your DM doesn't count encumbrance (with a low base strength, as most Druids have, every pound counts).
The purpose of this rule is because for many people finding non-metallic armour for a druid character is a fun sidequest.
I'm okay with that, and I also encourage players and DMs to do what's fun, but that sounds more like a good reason to place the line into their lore block and not weasel it into the proficiencies of their Class Features block.