Sage Advice doesn't in any way or form support this interpretation. Crawford merely explains why it is a rule. Having lore reasoning for the rule does not stop the rule being a rule. That should be blindingly obvious, as most rules in the game have some lore reasoning behind them.
Bit I can now certainly see how Sage Advice evolved from actually discussing the reasoning behind the rules to glibly reiterating the RAW back to people. If explaining the lore behind the rule and reminding people that GM can change the rules results people reading it as 'its not really a rule' then why bother?
And yes, I can actually see your agency issue, at least in theory. It would be better if the rule simply was some penalty for wearing the metal. But considering that the issue is unlikely to come up and the player chose this limitation themselves in the first place, I really don't see it as a big deal. And none of this has absolutely anything to do with whether it is a rule. There are a ton of rules in the game that could be formulated better, they're still rules.