So that doesn't mean using anything else to pay is illegal, it simply means I cannot force you to pay in anything other than the medium defined by law as legal tender. If you and I want to trade using seashells (which are anonymous, and can be traded anymously), that is not using legal tender but it most certainly is not prohibited by law.
Of course though if you declare seashells legal tender then you'd have a serious inflation problem due to their availability. This would make it necessary to pump so much acidic industrial waste into the sea that they all dissolve, thus re-valuing your currency.