That's not necessarily true and is in-fact a really bad way to describe how occluders should be used. Occluders really should be used in instances where it's impossible to otherwise block vis and force the engine not to be rendering models.
In your example of displacements, if it's a large enough displacemnet to block someone's view it's large enough to fit a nodraw brush under and then use hint brushes and other methods to guide VVIS accordingly.
I've never quite understood occluder use since people always say they can be expensive but useful it's a bit confusing. So Ive been looking in pl_upward decompile to see how they went about optimising for big open areas and they actually use quite a lot of occluders. Using the vis groups to mark them it says 43 objects selected which seems like a lot.
They use them in the obvious places, big func_details and big rock props etc. They use them in a few weird places I didnt expect as well though, inside solid brushes for example. I'm looking at the first point where blue comes out, looking towards the sniper/sentry nest building the top floor is func_details and props but the bottom floor is solid walls. Inside the solid wall there is an occluder. I assumed you could use hint brushes with solid brushes to block off visibility enough in a spot like that, I didnt realize putting occluders in solid brushes would help too but there they are. That is always a place with a lot of engy stuff and players hanging around so it makes sense I guess
Another confusing place is again at blue first spawn, when you come out the wall directly on your left has the final point on the other side of it because of the way the map spirals around. I assumed that whole area would be blocked off entirely as a seperate room so it wouldn't need extra optimisation, but the walls between the first spawn area and the final cap area are full of occluders/ I'm going to take a better look in game with the dev commands to see if it makes sense why they would do that, seems strange to me to put them there.