TL;DR: I'm sorry but what you are proposing doesn't look to be helpful to solve / reduce the "problem" depicted in the video / about the FAQ being too long, etc. Actually, the concept is already applied in multiple places around the site.
I took the screenshot that you posted in Meta SE and added a red hand drawn circle
Besides the issues already mentioned in the comments, it looks that you missed that already there is a banner that is shown to anonymous users:
This banner has the text
Super User is a question and answer site for computer enthusiasts and power users.
The above text is also in the site Tour, the article in /help/on-topic among other places.
Many of the questions that are not a good fit for this site come from users that do just pay enough attention to post what they whant to post, they don't care about the site culture and workings.