This plugin automatically disables default block editor for all post types and all users, without any settings or options. Just activate it and it works immediately.
By default, Classic Editor allows individual users to select whether they want to use block editor or classic editor on their profile edit screens. This feature must be disabled on Settings > Writing screen. For WordPress Multisite installations, this must be done on each site in the network, not network wide.
Additionally, Classic Editor stores post meta data in the database about type of editor that post has used.
On the other hand, Disable Block Editor disables block editor for everyone and everywhere, without any options or settings to configure, and without storing any data into the database.
Disable Gutenberg allows much finer control on where to and for whom to show block editor, and that is achieved by configuring settings. Basically, same differences as with Classic Editor.