That isn't to say that there are no issues with taking it to mean that way though, because it then opens up potential questions of how proficiency interacts with materials. We can ignore those questions, and assume that it doesn't matter in 99% of all cases, but it does make it a point that could be brought up.
I mean, if I am proficient with nonmetal shields, how does that make sense? What makes a metal shield function differently than a wooden shield? Does that mean that there has to be something for using a turtle shell as a shield. Likely not, but the question is brought up.
Forumers are turning a single awkward verb "will" into a mountain of unintended meanings and assumptions. Each assumption then provokes more questions, which in turn, adds soil to the mountain of meanings, that were never there in the first place.
Simply asking me, how can a Druid be non-proficient with metal, is itself, inviting me to answer, thus add to stories, that are all there because of one ambiguous word, "will".
The answer is, the Players Handbook doesnt explain why the Druid is nonproficient. It just states on three pages, that the Druid is only proficient with non-metal armor.
If you want me to invent an answer, that might be appropriate in some settings and not other settings, it might have to do with the fact that the Druid is spellcaster. Because the metal interferes with their magic, the class doesnt feel comfortable in it, and its tradition never learned how to use it.
The problem with the "interferes with magic" approach is, it is nonsense when considering the earth element loves metal.
An other explanation might be positive, rather than negative. The animals love Druids, want to protect Druids, and Druids gain protection from animal skins magically, and more easily, while focusing on druidic spellcasting mastery.
This "animals magically assist Druids" approach is probably more applicable. It explains how an earth Druid becomes proficient with animal armors while not even trying to become proficient with them. In other words, the animals have granted Druids proficiency with animal armors.
But again. The Players Handbook says nothing. Inventing mountain-loads of assumptions from a single verb seems misguided. The Sage Advice explanation, "because 1e Greyhawk", feels dislocated. And the Druid class mechanics and thematics seem incongruous generally.