I don't see the OP saying anything about "ai.txt" being for that? They're advocating it as a way that AIs could use fewer tokens to understand what a site is about.
(Which I also don't think is a good idea, since we already have lots of ways of including structured metadata in pages, but the main problem is not that crawlers would ignore it.)
In other words, there's no need to create an ai.txt when the robots.txt standard can just be extended.