Unfortunate but not unexpected
This is, indeed, disappointing, but not surprising. We must remember that life of any kind is, at its heart, a story about chemistry. If the chemistry isn't there, you can't have life.
But this does paint an increasingly confident picture about Mars that finds its first epoch a place of limited microbial possibilities, followed by four billion years of sterility. If there ever was life on Mars, its likely going to be decades before we ever know for certain.