Ya i suppose he could just draw a giant black pacman like texture over everything, one that has the cone part of it with a alpha of zero, might be costly dunno.
The shader though is small and will be fast with a single pass. Plenty of room to do more stuff as well, like dim out towards the edges or dim out as the visible distance increases or change the colors or whatever.
ya there are tons of shader examples on the forum here if you search for pixel shader.
writing-your-own-2d-pixel-shader-in-monogame-for-absolute-beginners