It's not clear to me how the Native File System API relates to the existing File System API.

https://developer.mozilla.org/en-US/docs/Web/API/FileSystem

Is Google proposing this as a replacement? As a supplement?

I made heavy use of the FileSystem API in a cross platform educational application a few years ago, tripping over a really hairy memory leak bug in Webkit along the way. By the end of that project the API had been deprecated, but apparently now it isn't any more and instead it's experimental? I have no idea what that means.