The low density/low height example in the nature article is still 5k people per Km^2. While definitions vary wildly, I usually see 1000-400 people per km^2 for suburb definitions.

Does example D look like suburbs to you? As something undefined it could be considered suburbs, but probably "streetcar suburb" in the Canadian/American context.

Critically the article also mentions a requirement for best practice greenery management to maximize carbon sequestration. I'm no botanist, but I'm guessing caretaken parks do better then monoculture lawns (assumption).

Edit: missed that the first link was a different study. That like on spanish cities has its lowest density group defined as <100 pop/hectare, if my math is right, that means <10,000 pop/km^2. Significantly denser than any suburb. This is also a region when thermal energy is spent of cooling, not heating. And while it adjusts for climate effect, it doesn't seem to adjust for the modernity/thermal effectiveness of the buildings. Such to say, a building with an air-conditioner will spend more thermal energy than one without.

Basically your two links are showing that cities can be too dense, and there is a point when they lose GHg efficiency. There is no mention of anything lower than what, as a Canadian, I would still call high density (just not super high density).