There is actually a science of studying Placebos. It is not as simple as most think. For example the strong the drug in many trials, the stronger the Placebo effect is. Things like the total number of trials of the drug become relative. Spatial Separation does not necessary imply Independence.
But... that is not at all what the linked essay is about? There's one paragraph in a 4.5k word essay explaining what a control group is that mentions the concept of a placebo because that's a concept related to control groups that the reader might be familiar with.
Here's the same author treating the subject of placebos more seriously: https://slatestarcodex.com/2018/01/31/powerless-placebos/
To prove the placebo effect exists, you need to have a group that thinks it's getting a placebo, but actually gets nothing...
To be clear, I'm not just making a joke, I think the ideas people have about the placebo effect are deeply incoherent and harmful.