It's not a shibboleth, it's a dog whistle. Shibboleths are for identifying people in one-on-one or small group situations. Aluminium vs. aluminum could be a shibboleth.

Dog whistles generally sound like platitudes to one group (family values, social justice, "let's have an honest conversation about race") and mean something entirely different to a particular demographic.

https://en.wikipedia.org/wiki/Dog-whistle_politics