An active core will release gasses to be accumulated on the surface of the planet through tectonic activity and volcanism. It will also generate a magnetosphere under the right conditions, which again shield those gasses from being blown away by stellar winds - but this is a very minor factor compared to the venting of gasses from an active core.

One more point against habitable planets around M class stars is that they are likely tidally locked due to the close orbits around the star - even in the Goldilocks zone. Maybe the biggest M class star could have a non-tidally locked planet in its GZ, but that would have to be a young star I guess.