I also don't see how they disprove the contribution of gravity. Remember that Earth is composed of fifty Titan-sized bodies.
Titan, and probably Uranus and Neptune, probably have their methane etc. as a result of outgassing - initially, the volatiles are embedded in the inner rocks, but as they gravitationally differentiate and heat - and are subject to tide-like interactions with other bodies - the volatiles are released.
(The real questions are "Why does Ganymede not have an atmosphere?" and "What's up with Venus, really?")
I believe the idea is that the water was all cooked out of Earth's protoplanetary disk material before it even formed large chunks. So gravity never got a chance to "contribute" on that front.
Titan, and probably Uranus and Neptune, probably have their methane etc. as a result of outgassing - initially, the volatiles are embedded in the inner rocks, but as they gravitationally differentiate and heat - and are subject to tide-like interactions with other bodies - the volatiles are released.
(The real questions are "Why does Ganymede not have an atmosphere?" and "What's up with Venus, really?")