I do not know whether it accounts for a negative savings rate, but the graph does show that people with disposable income were saving more this past year during the pandemic, which means they were spending less on consumer goods. There were probably two driving factors behind this: fear of economic instability due to the pandemic, and a lack of availability of paid activities due to the same. Presumably, consumer spending will rise again, as will travel and hence fossil fuel consumption per the original post.
The poorest X% not showing up in any statistics used for policy making is probably a longstanding problem... and it's probably hard to estimate by how much X has changed over the past year.
