Yeah, this. If you say to a player “your character can’t wear that chain shirt because they’re a druid,” that’s a house rule because the “rule,” such as it is, says “will not,” not “can not.” To be consistent with what it says, you would have to say “your character won’t wear that chain shirt, because they’re a druid,” which is a clear violation of player agency. If you allow them to wear it but impose some sort of penalty for doing so, you preserve player agency, but you’re creating a house rule to do so (and to be clear, there’s nothing wrong with that).
There are only two valid conclusions we can draw from these premises: either “druids will not wear heavy armor” isn’t a rule, or the rules of D&D 5e violate player agency. I can’t imagine the latter is the intent, especially given that Sage Advice clarifies that it’s just meant as a flavorful thing. Therefore I must conclude that it is not intended to be a rule.
If you want to follow the rules and have your PC wear metal armor, don't play a druid. No player agency harmed.