He didn't say that
You're pushing a strawman argument. He didn't say that life cannot exist anywhere but Earth. Few people say that. He didn't even say that life couldn't (or doesn't) exist on Mars. His post is based on the scientific community's assertion that the Viking lander experiments didn't find conclusive proof of life. The Viking tests were considered inconclusive because other things such as a chemical reaction with the soil could give a false positive result.
These new missions will be carrying better equipment to look for life. Who knows, they may find it. We can hope.