That anology doesn't cut any ice against quest markers. Presumably, the alternative to quest markers is directions given in dialogue. But directions are also the sort of thing that tell "people where exactly to go".
It is true that quest markers convey information not conveyed by directions. For instance, with quest markers, at any given moment you are not ignorant of the compass direction of the quest goal; with directions, you can easily be ignorant of this, if you are not keeping track of your location relative to the described directions. However, there's also information conveyed by directions that is not conveyed by quest markers. Directions typically involve a description of a specific path to take to the quest goal - follow this road, pass this landmark, and so on. Quest markers don't give you that information. Quest markers are silent on which path you should/can take, or what you might encounter on the way to the quest goal. So it's pretty implausible that quest markers "tell people where exactly to go" but directions do not. Rather, they both tell you that, but in different ways.
I think I completely agree with this.