[codethief]

> "gravity as a form of acceleration". […] The latter was Einstein's great insight. […] equating gravity and acceleration in the way Einstein did

This sounds like a gross misinterpretation of what Einstein said.

Einstein's great insight was that gravity defines inertial (i.e. non-accelerated) frames of reference. Put differently, in free fall you are not in an accelerated frame, despite what an outside observer on the surface of Earth might say. This is commonly abbreviated by "All massive bodies follow the same (inertial) trajectories in the presence of gravity, irrespective of their constitution", which is the Equivalence Principle.

What you are maybe referring to is what an observer experiences who stands still on the surface of a massive body: They can't free-fall, and so they're not in an inertial frame (= the thing defined by gravity) and thus actually accelerate upwards from the point of view of spacetime.

[n4r9]

I'm probably missing some of the nuances, but I'm referring generally to the Equivalence Principle, described by Einstein as:

> we ... assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system

[codethief]

I see. Personally, the way I have always interpreted that quote is that here, in this particular case, "gravitational field" is supposed to mean "someone standing on the surface of Earth, experiencing gravity" or, more generally, "an observer in a spacetime who's not in free fall / not moving along a geodesic" (but e.g. standing on the surface of a massive body).

Otherwise, as outlined in my previous comment, it does not make any sense.