Interesting thanks! The data I was looking at was apparently from 2018. So it seems the jump to "there must be a thick atmosphere" came from an assumption that the planet had as much iron as Earth?

Makes me wonder if B has an atmosphere but it's all frozen into a giant glacier on the night side.

This is actually pretty puzzling. B was thought to have the thickest atmosphere of all the planets because of its low apparent density. This could mean all the planets have underestimated densities, or maybe their composition on a whole is very different from our solar system.

Closest planet to star, so most transits, so most data to analyze.

They modeled several scenarios and this temperature almost exactly matches an airless black rock.

Sad. It would mean likely no atmospheres on Earth sized worlds around any red dwarfs, which are the most common kind of star.