Without further investigation it's hard to know what was done there.
Typically (in those days) one would go to the trouble of boxing in between the joists with plywood, but how far down the joists they went is anybodies guess. They would then fill-in between the joists and above with concrete and bring it up to the desired surface level. At which time the tile would be applied. In some cases the tile was tamped into place in that same wet mixture of concrete. In other cases the concrete was allowed to dry and then the tile was installed later using a Portland cement tile adhesive of some description.
What you have remaining may not go too deep and if crumbled wouldn't be worth leaving in place. Since you have a considerable distance to go to raise the floor back to it's original elevation you may be able to leave the concrete there for fill if it isn't all crumbles.
Another option may be to remove all the concrete to get rid of the weight, then install some plywood on top of the joists and go from there.
This is IF they haven't used a hatchet to carve away at the top edges of the floor joists, which was also a method used years ago to keep the wood joists from transmitting cracks through to the surface.