he took the Doppler Effect and using the Equivalence Principle, turned it into a Gravitational Shift. And then from there, he derived his eponymous Field Equations.

See

__The Essential General Relativity__.

By reversing this reasoning I've shown the following:

from the Gravitational Shift and using the Equivalence Principle, I've turned that into a Doppler Effect, and from there I then derived the Hubble Equation.

See

__The Equivalence Principle and the Big Bang Theory__.

So one inevitable conclusion is: either the redshift from faraway galaxies is the result of a Doppler Effect and that gives the BBT - or it's a gravitational effect and that gives the SUT. The second inevitable conclusion from the EP is that one interpretation cannot be differentiated from the other, that is, there are no conclusive tests that would favor one over the other. You either accept GR and the EP, or you reject both. Now note that only facts independent of GR/EP can tip the balance in favor of one of those models.