Well when I "suck it in" I can feel squishiness.
I doubt that it is all muscle. I have to remember that my body is freakish though. My appendix is located in the upper right quadrant of my abdomen and I've spent years living with IBS problems. IBS only rears it's ugly head at the onset of TOM, which I'm now finishing. Maybe TOM has something to do with it.
I guess my mystery fat hardness will remain a mystery for awhile. I'll keep working to see what softens up and falls off though.