There is a difference between he abs acting like armor and the legs (or arms) acting like armor.
The abs protect the internal organs. Thicker/harder abs (mostly a higher muscle tone) will less the impact of a blow to those internal organs and can thus have a real significant impact.
However leg and arm muscles are not the same way as a blow to those muscles will hurt almost the same regardless of the muscle mass you have.
I’m not saying that you should not make those muscles stronger, but that it will not have the protective effect that you think.