I highlighted the relevant bit on that sentence for you...
Makes sense this way?? If an instrument is not in a hazardous area and the signal isn't routed through a hazardous area, it doesn't matter if the variable displayed is from a hazardous area.
So, in that example, the instrument is in a hazardous area with rated box and likely intrinsically safe circuit with the correct parameters of the loop calculated.
But, perhaps the power supply powering the loop can't supply enough power to satisfy the requirement of having a gauge and instrument in the same loop, or maybe the gauge is to be on a wall of an office which is a safe area and therefore there's no need for it to be Ex rated. This being the case, a separate PLC output can drive that gauge without any regard for hazardous area requirements.