Does this apply even for iframes, or not?

Generally no - cross origin iframes don't allow camera/audio by default. Even if the toplevel site allows it (via https://developer.mozilla.org/en-US/docs/Web/API/HTMLIFrameE...), user still needs to grant permissions to toplevel site. Of course you can still use window.open and top.location.href in the iframe and use the same trick as in the article.