I don't think that's true. What is the alleged treaty called? The Antarctic Treaty binds the contracting states to prohibit individuals from doing various things in Antarctica, the Outer Space Treaty doesn't really do that, and arguably does the opposite by prohibiting territorial claims. Regardless, I don't think any treaty should be taken very seriously. They will both be violated whenever it's expedient to violate them.

Quoting the 1967 Outer Space Treaty, "States shall avoid harmful contamination of space and celestial bodies."

The 1960's definition of "harmful contamination" sure leaves a lot of wiggle room.

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.

As it should, though? At that point in time it was more of "we'll know it when we'll see it".

With that kind of wiggle room, it usually ends up being "We'll disagree when it becomes relevant".

I think parent is refering to wildly crazy nonexistent environment standards back then, like doing surface tests of nuclear weapons.

Remove the word ‘environmental’ and your statement gets better.

The treaty doesn't prevent Mars settlement. It just prevents claiming of territory. Antarctic treaty is actually significantly stricter than the outer space treaty in many ways.