Not quite. You can do a lot of stuff that requires no permissions, or at least not ones that the user has to confirm (e.g. you get internet permission, sensor access, always run in the background etc. by default, but you do need to declare this in the manifest file iirc), which isn't possible on websites like that (a website will ask before it lets a site do limited things while you think the tab is closed)

Depending on your threat model, it might be mostly harmless