That's still the same argument. It's possible Martian life has already been decimated by microbes from earth, but does that possibility justify carelessly introducing new extremophiles? There is a short window, less than a century and perhaps only a couple decades, before we start sending humans in which we can study what may have lived on Mars, originating from there or not, prior to colonization.

Conley is not trying to prevent Mars exploration or colonization. When we contaminate Mars, we should only do so as a deliberate choice, aware of the consequences, as there is no going back.