> The most likely source was a sheet of frozen methane beneath a rock formation

While it is certainly pretty likely, it isn't necessarily known that this rock is part of methane generation.

If it's coming from the ice directly, the ice sheet 'location' doesn't explain whether the methane was from a biotic or abiotic source.

It probably eliminates the already-small likelihood that it is coming from still-extant microorganisms, but it doesn't eliminate their possible involvement in clathrate formation.