The advise about 'destroying' watches you have is a bit odd too. Why not just return them or remove the listening in capability with a firmware update?
This is a legal thing: they decided that merely owning one of the affected devices is illegal. So the best way back to legality is to destroy the device and keep a proof of its destruction. Nice and tidy.
A firmware based fix alone does not make the device easily distinguishable from one without it and the fix is not permanent, making it possible to revert the device into its illegal state.
A firmware based fix alone does not make the device easily distinguishable from one without it and the fix is not permanent, making it possible to revert the device into its illegal state.