Here is what Chris McKay says about the question of isotopic ratios:
Source: Cassini: Making Sense of the NewsOriginally Posted by Quote from Chris McKay
The paper you quote mentioned the "Fischer–Tropsch reaction of the H2 with CO2".
How relevant this is, would depend what levels of CO2 and H2 there are in the bulk composition of Titan. If there is lots of CO2 subsurface, does that imply that any subsurface bodies of liquid water would be expected to contain lots of dissolved CO2? That would mean a more earthlike chemical situation subsurface than at the surface and in the atmosphere…