Why is it nonsense? Can it leave an area contaminated if it is not properly cleaned up?
Who the hell knows, since the British aren't making any chemical samples available to the international organization of chemical weapons experts, as per the Convention on Chemical Weapons to which both Britain and Russia are signatories.
Meanwhile Britain has advised anyone in the area of the Skripals that day to 'wash their clothes' as a precaution. Does that sound like the advice you'd give to someone if the chemical contamination remains present for many years?