As your source mentions, it's probably just accounting bullshit due to the gas transition.
Of course in the bigger scheme of things, it's not good due to being insignificant. If people actually want to make progress on climate change, we need to rapidly transition away from fossil fuels, and start drawing down greenhouse gasses, not emitting them. As a wealthy country, the US should be taking the lead on that front. Let me know when the fossil fuel corporations are dead, and US hits zero net emissions, that would be something to be happy about.
40 million tonnes seems significant to me. Although I will admit I'm sure I don't know as much about it as you. Realistically what would be a significant amount? Zero net emissions is not going to happen anytime soon.
As is usually the case, reductions should not be mentioned independently of the totals. The total US emissions for 2017 were apparently 5.14 billion metric tons, which makes 40 million tonnes fuck all.
If you start reading the science and what scientists are saying, it's beyond grim.
I think it's likely that the effects of anthropogenic climate change will essentially prune a few billion people and collapse global civilisation as we know it due to essentially irreversible destabilization of the world's weather. Perhaps it won't be quite that bad, perhaps it'll be worse and kill all of us apart from a few billionaires in their hi-tech climate-controlled bunkers.
From a sensible risk-mitigation point of view, we don't have any years left, and we don't have any more greenhouse gasses to safely emit. We not only need to reduce emissions to zero globally, we also need to draw all the extra greenhouse gasses out of the atmosphere as soon as possible.
Is it not good that we reduced by 40 million tonnes? That seem like something we should happy about.