This sentence says that consuming the biosphere is natural as gas release. This colours a choice as an unavoidable step, and thus justify it. And this kind of logic was used by nations against other (as a side issue, disagree with other opinion is OK, downvote it - no. unless it's a Parimate evolution thing).
You don't even have to see it as a choice. I mean, even if we ignore the present and the future, thus avoiding any kind of moral issues, we can look at the past and see what Homo Sapiens has already done. It did provoke the extinction of a whole bunch of large mammals and insular birds, deforested extremely large areas and many other things. To a large degree, most of the impact of our species on nature is already done, so we should be able to look at it objectively, as if we were talking about an other species (to some extent we are, considering how different we are from say our Mammoth-hunting ancestors). And when we do, we should not let someone say that this was the only extinction to occur because of a biological species, because that is just not true.
You can't pretend to protect nature if you refuse to look at it for what it is.
If someone states that 2+2 = 5, should I shut up and not state that 2+2 = 4 in case some people give this fact some bad moral interpretation?
They can, but they don't. I don't understand your point. The choice you're talking about is being made. It's not because you don't like it that it doesn't exist. In any case what matters is what is actually done, not what could be done in an ideal world.
So to me the impact of methanosarcina on the environment is indeed comparable to the impact of Homo sapiens, in the sense that both are extinctions induced by biotic agents.
So, to be able to think about the world more clearly the "neutral position" is invented. There's no moral, no good or bad, there's just stuff that happens, causes and effects. And from that point of view primate, making nuclear weapons and causing extinction of the species is no different from some bacteria, who filled atmosphere with metane.
But that, again, has nothing to do with what actual author of this thread was saying. So I'm saying it only to point out that your appeal to morality is invalid. And if you'd ask me, I don't want elephants to extinct, but that doesn't make neutral arguments bad. It might be my position that goes against neutral one, but speaking is not really of much value in that case.