Because email federation is inherent to everyone's understanding of how that service works. And perhaps more importantly, email "instances" are run by corporations. Laymen are not signing up on a "server" or "instance," they're signing up for Google, Apple, or Microsoft - the service they get aligns to a company that provides it. Nearly every single service that anyone has ever signed up for online has followed the same essential process: go to fixed url, create id and password, gain access.
It's easy to underestimate, especially in communities like this, how enigmatic the entire infrastructure of the internet is to the general population. Think of those videos where people are asked what "the cloud" is: they pause and ponder and then guess "satellites?" because they've never even wondered about it. I'm guessing that for many people, something like Twitter is just something that lives in their app store that they can choose to "enable" on their phone by installing it.
People know that software is "made up of code," but they don't understand what that means. The idea that an "application" is a collection of services run by code, that there are app servers and web servers, that there are backends and frontends, is completely unknown to (I'd guess) a significant majority of people. And if someone doesn't understand that, it's honestly near impossible to understand what anything in the fediverse is.
And most importantly: this is not any user's fault. IT and the Internet developed so quickly, and it was made so seamlessly accessible by corporations who at first just wanted their services to be adopted, and then wanted everything even more deliberately opaque so those users were more likely to feel locked in and dependent while the services themselves tail-spun in degradation.
We need more, and more accessible, and friendlier, tech literacy in general. The complexity of our world is running away from us ("I have a foreboding [of a time...] when awesome technological powers are in the hands of a very few, and no one representing the public interest can even grasp the issues" - Carl Sagan) and we simply can't deeply understand many of the things that directly impact us. But because of its ubiquity, IT may be the best chance people have of getting better at understanding.
I think the argument is that even if you're not consciously noticing it, your brain picks it up and that's part of the unsettling feeling you get. Is that true? I don't know. I was unsettled as hell watching Hereditary, but there's a lot more unsettling content besides the quasi-noticeable woman in the corner of the ceiling.
I'm glad to see It Follows included in the essay though. Watching characters converse in the grass, in the sunlight, in a scenario that in almost all other horror movies would be a tension-relieving safe scene, until you notice another character, blurry and deep in the distance, walking robotically on a straight path toward the central characters, still gives me chills. It remains one of my favorite effects, and is a top-tier reason why I love horror movies even though I don't love feeling tense or scared.