Guardsman (Valspar) do not actually state that you have to only use Safeclean (also owned by Valspar) to clean furnishings under the policy - if I remember rightly?
However, if it was proven that the item has been badly cleaned by someone else then I suppose they could feasibly 'wriggle' out of responsibility saying that may have affected the protector etc, but they would never know this unless the custy actually told them of a bad clean :

, or if a safcelean technician could poss tell this when on a stain removal visit.
Their policy wording is pretty tight though and covers all angles - as you would expect from a national/international company.