Methane doesn't come from volcanoes, or from organic life; it comes from primordial deposits of hydrocarbons from the formation of the planet, released by heating of the deposits or sublimation, or whatnot. If there is life, and it is carbon based, then it subsists on these deposits and the evolved methane might be a waste product. I think the speculation is way off in the stratosphere here. I think further analysis is called for before drawing any conclusions.

Further, there is always this to consider: http://www.nickbostrom.com/extraterrestrial.pdf