Only the presence of CO2 and water does not say anything about conditions favorable for life.

These are some of the most abundant substances in the entire Universe. On any medium-size planet without life, the atmosphere is expected to be composed mostly of CO2, together with a smaller amount of dinitrogen, like it is today on Venus. Also the rarefied atmosphere of Mars contains mostly CO2.

Because most rocks containing carbonates and sulfates are formed by precipitation from water, this discovery is just additional evidence that in the past Mars had much more water than today, so that there was some water lake or sea where these rocks are located now.

The atmosphere of the early Earth must have been composed mostly of CO2 too, before algae have appeared and they have reduced most of the CO2 to organic compounds, leaving an atmosphere where N2 is the most abundant component.