That's true, but it's defined (and kept up to date) in practice by COSPAR, who have a pretty elaborate set of definitions in place for Mars in particular.
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.
The rules for Mars aren't based in the treaty but instead on "planetary protection" an abstract policy coming from no law other than forms of regressive environmentalism that tries to limit contamination so that it's easier to find potential microbes. It's already actively harming site selection for Mars exploration by preventing sending of rovers that might discover life to actual areas that might have life.
To save the possibility of finding life we are preventing ourselves from finding life.
