The video doesn't do a great job of explaining everything that's going on. It's a bit more than just bundling Trix into Rails core. It seems like it's doing a lot of processing of the content fed into it.
I've done quite a lot of custom CMS work with Rails and it's interesting to see their approach. It's not too far off from stuff I've done before, which I'll take as validation :)
There is some Trix specific code for attachments, but I don't see why you couldn't swap in some of your own handlers if you wanted. Just a few months ago I wrote some JS plugins to hook CKEditor5 file uploads to ActiveStorage, along with a hacky Nokogiri parser to associate the attachments. I would have loved to be able to just hook into something like this.
A vast majority of web apps deal with rich content in some fashion. In that light, I think having a framework for storing/parsing/etc with it is quite justifiable, especially at the stage Rails is at. That it comes with a default UI that handles many use cases is really just icing.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.