Cosmic Microwave Background (CMB) Radiation is known to be thermal to very good approximation. This means that when it was created, this radiation has already been thermal. This is because the outcome of stretching a thermal spectrum (e.g. by metric expansion of space) is a thermal spectrum again.
The origin of the CMB was re-combination of H and He plasma to neutral atoms as it cooled, and that turning from a foggy plasma to a transparent gas "released" the radiation.
What I am confused about is that this radiation was already black-body and has no imprint of spectral lines from neither hydrogen nor helium. Why that? Or where is my thinko?
Note: There is this answer https://physics.stackexchange.com/a/391849/251841 that just re-states that the radiation was black-body from the start without explanation why / how effects of the electron shells (line spectra) were cancelled out.
Even when there's not enough energy to ionize an atom, there should still be enough energy to make electrons transit from one shell to another?