The author is comparing cooking between different types of stoves, not indoor air quality between different activities. I believe you would call this a controlled variable, for example by making sure you cook the same meal, at the same quantity, the same way. Real scientists can correct me.
You might want to take a minute to read the article itself. The author measures NO2 levels in their house with the equipment (stove, furnace, etc) that they have and the meals that they cook (or don't, e.g. when getting takeout). They make no comparison to other stoves.
There is an implicit comparison through the research he cites. It’s also almost certain the reader would be using this information to decide on moving away from gas. No one is trying to use this information to make a decision on whether to abandon cooking. They want to know how bad gas stoves pollute indoor air so they can weigh their decision to go electric.
Sure. But the comment you responded to was about the observational study that the author did, not about the research. The question is about what portion of the observed emissions come from cooking per se, as opposed to the gas that is used to do the cooking. I suspect it is a small portion, but it is still an interesting one, and not addressed by the article.
And that comparison hasn’t been made. Sure. The food emissions are going to be (about) the same for the same meal, regardless of the stove. But nobody knows whether the food emissions make up 5% or 95% of the emissions in his measurements. Because he hasn’t compared his gas stove to any other stove.
