I can't say about other companies but I have personally done many, many of these interviews over the years and can say with 100% confidence that someone who has spent days/weeks/months studying system design interview material but not had any actual experience building such systems in a production setting will get weeded out in the first few minutes.
Now some interview prep might actually help a lot of people, but it will help you better present your existing knowledge, not replace it.
I may be a counter example, but I did very well at system design interviews at both Google and Meta (level L5), with no experience building such systems. As a matter of fact, my resume didn't mention any such things and everybody was aware I didn't have that experience - recruiters, interviewers. A lot of candidates don't have that background (there are lot of things to do in our field besides building Twitter newsfeed and Google search autocomplete), and companies don't want to exclude them.
Also, you can build knowledge during your preparation. It's not only about "faking" it.
Now some interview prep might actually help a lot of people, but it will help you better present your existing knowledge, not replace it.