One frustrating aspect to the study is that it was hard to determine whether they are comparing like for like per unit time. They say the “operation of a gas stove” and “running a generator” — but for how long? It doesn’t seem like they tested each of these things under similar conditions in their lab but rather relied on other studies for that data. Figure 2(b) right does seem to measure this but they haven’t labeled the chart with clear labels and the description is a bit ambiguous.
After reading the study, I think the issue is that the claim it is making is slightly different than the one in the headline. They are measuring VOC and ~PM2.5 pollutants, but gas engines (and gas stoves presumably as well) produce other pollutants like CO, which is what kills you of you run a gas generator indoors.