[AndrewC]

When life first began on earth the atmosphere was primarily methane and ammonia.

Sorry, that formula has been updated. The early earth atmosphere was not a reducing atmosphere. It consisted mainly of CO₂, H₂O, and N₂. Reflections From a Warm Little Pond

Now the Oxygen signal(free oxygen, I presume that is what they are talking about in this article) no longer signals life.

From the article ---- Despite the oxygen, the faraway planet is not one that would support life.