Did this paper just argue that there is surely life on Titan?
The underlying assumption in the paper is that the planet has water in liquid form. 

The paper suggests that finding both CO2 and CH4 in the atmosphere of a rocky planet in the "habitable zone" (in which water can exist in liquid form) would be hard to explain, without life. Thus one doesn't need to detect atmospheric oxygen. If you can detect both CO2 and CH4, this should be taken as evidence of anoxic life (probably in the form of microbes) 

Titan has CH4, but it doens't have much CO2 and there is no liquid water.
Titan is not in the habitable zone. On Titan, H2O is a forms part of the "rocks", and the oceans are made of hydrocarbons. Titan is not a good location to look for life as it doesn't have free liquid water. We have a photo from Titan's surface and it isn't covered in Microbes