Yes that part is thick rubber.
FYI I also own the first iteration of these (white sole on black leather) for yrs. As long as you vibram them every year, they last as long as any work boot.
Re your question re leather thickness, it all depends obviously on the type of leather.
I find when simone uses horse (I own Augusta horse derbies, horse workboots, new horse sneakers) it is thicker and stiffer than CCP. The leather on my Augusta workboots and derbys (pics earlier in...