  Song and Bernevig (SB) have recently proposed a topological heavy fermion
description of the physics of magic angle twisted bilayer graphene (MATBG),
involving the hybridization of flat band electrons with a relativistic
conduction sea. We explore the consequences of this model, seeking a synthesis
of understanding drawn from heavy fermion physics and MATBG experiments. We
identify a key discrepancy between measured and calculated onsite Coulomb
interactions, implicating renormalization effects that are not contained in the
current model. With these considerations in mind, we consider an SB model with
a single, renormalized onsite interaction between the f-electrons, containing a
phenomenological heavy fermion binding potential on the moir\'e AA-sites. This
feature allows the simplified model to capture the periodic reset of the
chemical potential with filling and the observed stability of local moment
behavior. We argue that a two stage Kondo effect will develop in MATBG as a
consequence of the relativistic conduction band: Kondo I occurs at high
temperatures, establishing a coherent hybridization at the $\Gamma$ points and
a non-Fermi liquid of incoherent fermions at the moir\'e K-points; at much low
temperatures Kondo II leads to a Fermi liquid in the flat band. Utilizing an
auxiliary-rotor approach, we formulate a mean-field treatment of MATBG that
captures this physics, describing the evolution of the normal state across a
full range of filling factors. By contrasting the relative time-scales of
phonons and valence fluctuations in bulk heavy fermion materials with that of
MATBG we propose a valley-polaron origin to the Coulomb renormalization and the
heavy fermion binding potential identified from experiment. We also discuss the
possibility that the two-fluid, non-Fermi liquid physics of the relativistic
Kondo lattice is responsible for the strange metal physics observed in MATBG.
