> 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.

The GP's question was rhetorical and his point was that methane can come from both biotic and abiotic processes.

> If you read the article, they explain where the methane is coming from.

This is technically true in a spatial sense, but not at all true in reference to the GP's point. The article specifies only the location of the source, not whether the source is biotic or abiotic.