>light is assumed to not interact with gravity in that framework in any way.

Light absolutely interacts with gravity in Newtonian mechanics. One of the neat features of Newtonian gravity is that gravitational charge (which is mass) equals inertia so the two cancel each other out exactly in the equation of motion. No other force has this property.

This means gravitational acceleration is independent of inertia, even in the case where inertia/mass approaches zero.