If your floor is constructed the same as mine, it is 2 layers of 7/8th inch OSB extending to the outer edge of the wall laid over a framework made of 1 1/2" square steel tubing. There is 1 1/2" Styrofoam insulation inside the steel frame. Under it is the woven black plastic fabric layer which is the ceiling of your compartment.
The bolts you are seeing would likely be above the insulation, possible with some sort of backing plate.
During assembly, the wall is set on top of the OSB and bolted through to the 1 1/2" x 1 1/2" steel frame sandwiching the OSB in place.
I have drilled through in that general area to secure a large recliner which the previous owner had sitting loose.
Drilling through the OSB is straightforward. I would suggest doing it from underneath, measuring in about 2" and going straight up.
If you are doing it from above it is difficult to drill tight to the wall without sloping into the steel framing although it is behind your couch so it probably does not matter if you move the hole out a little bit.