Would Titanians(?) say: "What a big, hot, toxic planet that distant Earth is, so close to the Sun. It's 2/3 covered with molten rock - obviously uninhabitable!"
Size perspective: I always see things like "Tethys is 1/3 the size of Earth's moon". Or "Mars is 1/2 the size of the Earth." That's like saying a tennis ball is 1/3 the size of a soccer ball. Thus we get distorted ideas of how big worlds are. Volume - the true measure of "size" - is proportional to the cube of the diameter.
So in round numbers, Earth (12756Km diam.) is 6-1/2 times the size of Mars (6787Km). Titan (5156Km) is 1/2 the size of Mars. Tethys (1060Km) is 3% of the size of Earth's moon (3476km) or 1% of the size of Titan.
...Of course, if we want to know how many acres it has for golf courses, surface area is proportional to the square of the size.
That the H2 is more an analog to O2 than to CO2, ie that it's a breathing/energizing gas for animal life, is indeed what I was saying.
And the reaction, C + 2H2 --> CH4, should tend to replenish Titan's methane.
Visually though, from space one sees the Amazon rainforest canopy without seeing any of the animals in it. Plant life will be far more evident than animal life.
If the H2 is analogous to CO2 rather than O2, then vegetation taking it up could explain its absorption at the surface pretty nicely.
On the other hand, it would seem H2, like O2, will easily react to release energy, whereas normal (unstretched) CO2 is usually an 'end product' of reactions, hard to react further to get energy out of. On Titan I expect that 'end product' would be methane(?):
C + O2 -> CO2 (solid on Titan)
C + 2H2 -> CH4 (liquid)
In the absence of another energy gas on Titan, I'm expecting H2 would be analogous to O2, though it may be there's no direct parallel - H2 appears to be being absorbed rather than produced (by plants) at the surface judging by the findings.
That explains all!
0.1% is a lot less than 20+% O2 on Earth, but it seems the best bet on Titan for a breathing/energizing gas as far as I've seen.
In the 2005 article "The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe", I was puzzled to see no mention of H2 actually being present in the atmosphere, given previous estimates of 0.5% and feeling it was likely to be biologically important, perhaps a breathing gas. It was indirectly implied, but I was left with the impression it must be pretty insignificant.
Just how much H2 was actually found, say, near the surface and at a given altitude or two? PPB? PPM? whole %s?