Confusingly, according to the MDN document I linked, that permission is not needed.
Clipboard APIs are in general pretty permissive in writing, any websites can write anything to the clipboard without requesting any permission, if it's done within like 1000ms or so from user interaction. So you don't even need an extension to write to clipboard.
https://github.com/ClearURLs/Addon/blob/master/external_js/c...
I personally do not know, just going by what's stated within the URL in the adjoining comment (https://github.com/mdn/webextensions-examples/tree/master/co...).