Read your link again. That sentence, the only one which says harmful contamination, refers to the earth, And only the earth. It is very explicit about that. I don’t know what else to say.

Not to wade into your fight, but danaliv is right. Here is (most of) the sentence:

“States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter....”

The “their harmful contamination” part clearly refers to something plural (the moon and other celestial bodies). The second part would be redundant if those were sources, rather than recipients of contaimination.