C'mon, that's simply not true. The essential innovation was present in neither of those games. In GE, different weapons have different loudness values. As you fire more shots in rapid succession, a multiplier is applied, increasing the radius of the sound. So in Facility and Bunker, the player is encouraged to use their silenced PP7 and space out their shots. Firing rapidly, or using unsilenced weapons, will attract AI from nearby rooms. This does depend on portal/room placement, though. Pretty much every modern stealth FPS uses some version of this system.