I think they said what _they_ meant _and_ still meant what _you_ said.

Is this a common joke?

Thanks, I've learnt something today.

Just out of curiosity: is it just a joke, or is there an idiocy behind, such as Sovereign Citizen or similar?

It's tongue in cheek or wink wink. No one uses it seriously but there is sometimes an undercurrent of resistance to what's seen as an external cultural standard. But you'll never see it used as a kind of rallying cry.