Which suggests that this is part of the long-term geologic scale climate self-regulation - the methane stays locked in the seabed, regardless of water temperature, until there is a sufficient freeze that a significant amount of ocean water becomes ice on land, at which point the methane is released due to the decrease in water pressure and, once released, helps to re-warm the atmosphere to counteract the ice age that led to the freeze in the first place. The released methane doesn't stay in the atmosphere forever, however, because it ultimately gets broken down into CO2 and water vapor.
That was my figuring.
Considering there have been multiple glacial Epochs and interglacial periods just since the Cambrian, some sort of self-regulating mechanism is likely present to prevent "Iceball Earth".
The Methane Hydrates are my first and best suspect. The more ice cover, the higher the albedo, so some other mechanism than raw variations in insolation would likely be present, and at the point where ocean levels were reduced enough, the Methane Hydrates would become unstable and de-gas, providing a mechanism for warming independent of surface absorption of energy, and reversing the glaciation.
Incidentally, that is why I am not in favor of using those same Methane Hydrates as an energy source.