That link you posted is referring to a completely different API, the File System Access API. There are a bunch of different APIs with extremely similar names in this space, with very different levels of browser support.
1. FileSystem API: (Note that this is one word, "FileSystem.") This deprecated API simulates a local file system that web apps can navigate around. With this API, you can develop apps that can read, write, and create files and directories in a sandboxed, virtual file system, but you can't use it to access files in the user's Documents folder.
You can polyfill this API, and IMO it's not very useful. It's typically backed by actual files, but there's no expectation that users will directly access that folder.
https://caniuse.com/filesystem
https://caniuse.com/mdn-api_filesystem
2. File System Access API (formerly known as Native File System API and prior to that it was called Writeable Files API): This API lets webapps launch a picker to select a file or folder; from then on, the webapp can write to that file/folder without prompting the user (until you close the tab).
Without this API, if your webapp has a file you'd like to write, all you can do is let the user download it. Downloaded files always either automagically appear in the Downloads folder (and do not permit overwriting existing files; identical names get autorenamed), or they launch a picker, which nags the user with "Are you sure your want to replace this file?"
The FSA API cannot be polyfilled, and it's impossible-ish to implement a good web-based text editor without it. (Imagine if your favorite editor could only generate new files and never overwrite old ones, or if it nagged you "are you sure?" every time you tried to save your changes.)
That's the API you linked to above. There, a guy says he thinks it should be harmful, but that's not the standards position that Mozilla actually adopted. They currently mark it as "defer" (though they may decide to come around and mark it "harmful") https://mozilla.github.io/standards-positions/#native-file-s...
https://caniuse.com/native-filesystem-api
3. File Handling API: This is what TFA is about. It allows your webapp to say "I handle .txt files; when someone double clicks on a .txt file, open it in my webapp."
Mozilla lists this API as "defer" as well. https://mozilla.github.io/standards-positions/#wicg-file-han...
https://github.com/mozilla/standards-positions/issues/158 is where they discussed that. (Note this is issue 158, as opposed to issue 154 in your link.) I agree with Mozilla that it makes no sense to implement the File Handling API without the File System Access API.