Earlier quoted context omitted.
A non-state actor is possible, but unlikely. It would require an organization, a small group at least, that had the inclination, opportunity, skill and resources. Any one or two of those is easy, but to actually do it requires all of those simultaneously. Furthermore such a serious attack from eco-terrorists would be fairly unprecedented. It's a far cry from torching a resort or tree spiking.
Releasing colossal amounts of methane directly into the environment would be an odd choice for an eco-group too.
Probably, but there conceivably might be some 'ends justify the means' logic that could motivate it. There have been some eco-terrorist attacks against nuclear power plants for instance, which on the face of it seems contradictory but I presume is justified using some sort of "raise awareness" logic. But without any eco-terrorist group taking credit for this, such a motivation seems particularly unlikely.