In the Titan atmosphere hydrogen is in dynamic equilibrium. It is being continuously created by uv splitting of hydrocarbons and it is being continuously consumed by reaction with other gases in the atmosphere.
In NASA's models of the atmosphere the equilibrium concentration of hydrogen turns out to be roughly independant of height. I'm not familiar with their models, but this is presumably because the ratio of the production and reaction rates is roughly independant of height so there is no net flux of hydrogen in the atmosphere. I'm sure their model includes upwards diffusion of hydrogen :-)
Now assuming that all the physical parameters in NASA's model are correct, if a concentration profile with height is observed it must mean there is either a source of hydrogen or some process consuming hydrogen that isn't included in the model. The suggestion is that there is some process near the surface that consumes hydrogen, and that one possible process is metabolism by living organisms.
We'd all love there to be life on Titan, so it's very tempting to use the apparent deficit of hydrogen near the surface to jump to this conclusion. But there must be loads of other possible mechanisms, and there must be a good chance it's just down to a purely physical process not included in the atmosphere model.