Why would it need subatomic physics? Seems like thermodynamics is the key here.

Yes, basic thermodynamics solves the problem and subatomic physics is not strictly necessary.

SAMU was still discussing albedo, which he argues is irrelevant to thermodynamics. Subatomic physics is relevant to the physical phenomenon of reflection, expressed in the macro effect as albedo, because reflection is partially governed by the wavelength of the incident light in combination with the subatomic properties of the surface.

For thermodynamics purposes, where comparison to the blackbody absorber/emitter is encapsulated relatively simply without loss of generality, the subatomic component of light reflection is far more detail than is necessary or helpful.

I wasn't necessarily agreeing to all the implications of SAMU's laundry list of relevant fields, only suggesting that a more complex approach is in order.