The link above is to a different section of the page, giving the mistaken impression that this is unfettered access to the local file system, which it isn't. Quoting the appropriate section further down the page:

The origin private file system (OPFS) is a storage endpoint private to the origin of the page, providing optional access to a special kind of file that is highly optimized for performance, for example, by offering in-place and exclusive write access to a file's content.

Storing data in the OPFS is similar to storing data in any other browser-provided storage mechanism that's private to the origin of the page (for example the IndexedDB API). This means that files in the OPFS differ from files selected using a picker in the following ways:

- Permission prompts are not required to access files in the OPFS.

- Clearing data for the site deletes the OPFS.

- The OPFS is subject to browser quota restrictions.