Yep. For me 75 meters of RVR are enough to land in cat IIIB. I heard that the 75 meters RVR are there to enable the pilots to vacate the runway AFTER landing, and taxi to the gate. The landing itself is irrelevant (as there is no DH)
Cat IIIC would be 0 RVR. This would mean you need an auto coupled guidance to the gate. this does not exist as we all know. But some time ago when the system was invented and all these different ILS integrities were made, it was contemplated.
It was considered too expensive, so hence there is no cat IIIC. (0 RVR requirement)
I guess the landing in prague was completely legal.