It’s more like, you wouldn’t put guns in a sword fighting game unless you disadvantage them in a way to still be fair. That’s just balancing. And balancing can have a lot of different shapes and forms. Speed is one way. Works for guns in sword games (flintlock guns are naturally slow to reload so you can believably do that in a period setting) and to some extent for snipers.

Map design would just be another way of balancing. Games are always designed around their mechanics (or at least good ones are). Super Mario wouldn’t be fun if you could just fly to the end of the level. If you put obstacles in the air as well though it’s balanced again. You change the design of the level to fit the gameplay. And in a game that has a somewhat powerful sniper, you don’t design a map with an impenetrable sniper nest that can overlook the whole map.

That’s kinda what the pawns are tho

Chess needed several things basically like that, though. It's why you can castle, move pawns once or twice on their opening move, and en passant.

Is advocating for good map design about designing the whole game around it, or is it just balancing the game?

Plus, game developers should be designing their entire game around what’s in it, that leads to balanced, cohesive games. A shooter with bad maps is a bad shooter.