COSPAR is (officially speaking) a private body, and as such their Planetary Protection Policy is not legally binding. They propose their policy as an interpretation of Article IX of the Outer Space Treaty, but as a private body they lack the legal authority to make binding interpretations of an international treaty. The standard approach in international law would be to look at the intentions and understanding of the States Parties at the time the Treaty was originally concluded in order to interpret it - which would likely support a far weaker understanding of “harmful contamination” than what COSPAR proposes.